📝 Update README

Author: gabe565

README.md

@@ -21,3 +21,97 @@ docker pull ghcr.io/clevyr/yampl
 ## Usage
 
 [View the help docs for usage information.](docs/yampl.md)
+
+## Templating
+
+
+
+### Functions
+
+All [Sprig functions](https://masterminds.github.io/sprig/) are available in templates, along with some extra functions:
+
+### `repo`
+
+Splits a Docker repo and tag into the repo component:
+```gotemplate
+repo "nginx:stable-alpine"
+```
+The above produces `nginx`.
+
+### `tag`
+
+Splits a Docker repo and tag into the tag component:
+```gotemplate
+repo "nginx:stable-alpine"
+```
+The above produces `stable-alpine`
+
+### Variables
+
+All variables passed in with the `-v` flag are available during templating.
+For example, a variable given as `-v tag=latest` can be used as `{{ .tag }}`.
+
+The previous value is always available via `.Value` (`.Val` or `.V` if you're feeling lazy).
+
+### Example
+
+Here is a simple Kubernetes nginx Deployment:
+
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nginx-deployment
+spec:
+  selector:
+    matchLabels:
+      app: nginx
+  template:
+    metadata:
+      labels:
+        app: nginx
+    spec:
+      containers:
+        - name: nginx
+          image: nginx:1.20.2 #yampl nginx:{{ .tag }}
+          ports:
+          - containerPort: 80
+```
+
+In this example, notice the yaml comment to the right of the `image`.
+
+If this file was called `nginx.yaml`, then we could replace the image tag by running the following:
+```shell
+yampl -i nginx.yaml -v tag=1.21.6
+```
+
+The file would be updated in-place and would end up looking like:
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nginx-deployment
+spec:
+  selector:
+    matchLabels:
+      app: nginx
+  template:
+    metadata:
+      labels:
+        app: nginx
+    spec:
+      containers:
+        - name: nginx
+          image: nginx:1.21.6 #yampl nginx:{{ .tag }}
+          ports:
+            - containerPort: 80
+```
+
+If I wanted to repeat myself even less, I could utilize the `repo` function to pull the existing repo through.
+I could define the `image` template as:
+```yaml
+image: nginx:1.21.6 #yampl {{ repo .Value }}:{{ .tag }}
+```
+
+This would generate the same output, but I didn't have to type `nginx` twice.
+This becomes more useful when using custom Docker registries where repo names can get quite long.