docs: add plugin docs

docs/plugin/Makefile

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+.PHONY : test build clean format
+
+build:
+	go build -buildmode=plugin -o templateplugin.so

docs/plugin/README.md

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+## Tob Plugin
+
+https://pkg.go.dev/plugin
+
+You can create custom functionality for services. This is needed when you want to create a service that is not currently available on Tob, or when you want to create a custom message that will appear on the monitoring Dashboard or a message that will appear on the Notificator.
+
+There are several limitations currently at https://pkg.go.dev/plugin. 
+- Binary Plugins must be built with the same code as the code you use to build the main binary.
+
+- The version of the operating system used to run the plugin binary and main binary must be the same as the one you used to build the plugin binary and main binary. For example, if you want to run the plugin binary and main binary on Ubuntu 20.04, you have to build it using Ubuntu 20.04.
+
+### Getting started
+
+- Clone the latest Tob code
+```shell
+https://github.com/telkomdev/tob.git
+```
+
+- create your plugin folder, for example `dummyplugin`
+```shell
+mkdir dummyplugin
+```
+
+- copy the `templateplugin.go` and `Makefile` into your `dummyplugin` folder
+```shell
+cp docs/plugin/templateplugin.go dummyplugin/
+cp docs/plugin/Makefile dummyplugin/
+```
+
+- build the binary
+```shell
+cd dummyplugin/
+make build
+```
+
+- add configuration to your `json config`, by adjusting 2 fields. The `kind` field is filled with `plugin`, and `pluginPath` is filled with plugin files with the extension `.so`.
+
+```json
+"dummy_plugin_one": {
+    "kind": "plugin",
+    "url": "https://www.google.com",
+    "checkInterval": 5,
+    "enable": false,
+    "tags": ["product 1", "product 2"],
+    "pics": ["bob", "john"],
+    "pluginPath": "/home/john/tob/dummyplugin/dummyplugin.so",
+    .....
+```

docs/plugin/templateplugin.go

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+package main
+
+import (
+	"log"
+	"math/rand"
+	"time"
+
+	"github.com/telkomdev/tob"
+	"github.com/telkomdev/tob/config"
+	"github.com/telkomdev/tob/util"
+)
+
+func main() {}
+
+// TemplatePlugin service
+type TemplatePlugin struct {
+	url               string
+	recovered         bool
+	lastDownTime      string
+	enabled           bool
+	verbose           bool
+	logger            *log.Logger
+	checkInterval     int
+	stopChan          chan bool
+	message           string
+	notificatorConfig config.Config
+}
+
+// NewDummy Dummy's constructor
+func NewTemplatePlugin(verbose bool, logger *log.Logger) *TemplatePlugin {
+	stopChan := make(chan bool, 1)
+	return &TemplatePlugin{
+		logger:  logger,
+		verbose: verbose,
+
+		// by default service is recovered
+		recovered:     true,
+		checkInterval: 0,
+		stopChan:      stopChan,
+	}
+}
+
+// Name the name of the service
+func (d *TemplatePlugin) Name() string {
+	return "plugin"
+}
+
+// Ping will try to ping the service
+func (d *TemplatePlugin) Ping() []byte {
+	n := rand.Intn(100)
+	if n < 50 {
+		d.SetMessage("dummy plugin has an error")
+		return []byte("NOT_OK")
+	}
+
+	return []byte("OK")
+}
+
+// SetURL will set the service URL
+func (d *TemplatePlugin) SetURL(url string) {
+	d.url = url
+}
+
+// Connect to service if needed
+func (d *TemplatePlugin) Connect() error {
+	if d.verbose {
+		d.logger.Println("connect dummy")
+	}
+
+	return nil
+}
+
+// Close will close the service resources if needed
+func (d *TemplatePlugin) Close() error {
+	if d.verbose {
+		d.logger.Println("close dummy plugin")
+	}
+
+	return nil
+}
+
+// SetRecover will set recovered status
+func (d *TemplatePlugin) SetRecover(recovered bool) {
+	d.recovered = recovered
+}
+
+// IsRecover will return recovered status
+func (d *TemplatePlugin) IsRecover() bool {
+	return d.recovered
+}
+
+// LastDownTime will set last down time of service to current time
+func (d *TemplatePlugin) SetLastDownTimeNow() {
+	if d.recovered {
+		d.lastDownTime = time.Now().Format(util.YYMMDD)
+	}
+}
+
+// GetDownTimeDiff will return down time service difference in minutes
+func (d *TemplatePlugin) GetDownTimeDiff() string {
+	return util.TimeDifference(d.lastDownTime, time.Now().Format(util.YYMMDD))
+}
+
+// SetCheckInterval will set check interval to service
+func (d *TemplatePlugin) SetCheckInterval(interval int) {
+	d.checkInterval = interval
+}
+
+// GetCheckInterval will return check interval to service
+func (d *TemplatePlugin) GetCheckInterval() int {
+	return d.checkInterval
+}
+
+// Enable will set enabled status to service
+func (d *TemplatePlugin) Enable(enabled bool) {
+	d.enabled = enabled
+}
+
+// IsEnabled will return enable status
+func (d *TemplatePlugin) IsEnabled() bool {
+	return d.enabled
+}
+
+// SetMessage will set additional message
+func (d *TemplatePlugin) SetMessage(message string) {
+	d.message = message
+}
+
+// GetMessage will return additional message
+func (d *TemplatePlugin) GetMessage() string {
+	return d.message
+}
+
+// SetConfig will set config
+func (d *TemplatePlugin) SetConfig(configs config.Config) {
+
+}
+
+// SetNotificatorConfig will set config
+func (d *TemplatePlugin) SetNotificatorConfig(configs config.Config) {
+	d.notificatorConfig = configs
+}
+
+// GetNotificators will return notificators
+func (d *TemplatePlugin) GetNotificators() []tob.Notificator {
+	return tob.InitNotificatorFactory(d.notificatorConfig, d.verbose)
+}
+
+// Stop will receive stop channel
+func (d *TemplatePlugin) Stop() chan bool {
+	if d.stopChan == nil {
+		d.stopChan = make(chan bool, 1)
+	}
+
+	return d.stopChan
+}
+
+// Exported
+var Service TemplatePlugin