Fix ci-build status badge and link status badge to actions (#99)

* Fix badge for ci_build.yml workflow on master branch.

The URL format didn't match the [currently documented badge URL](https://docs.github.com/en/actions/monitoring-and-troubleshooting-workflows/monitoring-workflows/adding-a-workflow-status-badge#using-the-branch-parameter).

* Link to ci-build runs on master branch instead of to SVG of badge.

* Only include ci_build.yml action, rather than everything on master branch

Author: drichardson

README.md

@@ -1,6 +1,6 @@
 # PyTorch learning rate finder
 
-![](https://github.com/davidtvs/pytorch-lr-finder/workflows/ci-build/badge.svg?branch=master)
+[![ci-build status](https://github.com/davidtvs/pytorch-lr-finder/actions/workflows/ci_build.yml/badge.svg?branch=master)](https://github.com/davidtvs/pytorch-lr-finder/actions/workflows/ci_build.yml?query=branch%3Amaster)
 [![codecov](https://codecov.io/gh/davidtvs/pytorch-lr-finder/branch/master/graph/badge.svg)](https://codecov.io/gh/davidtvs/pytorch-lr-finder)
 [![](https://img.shields.io/pypi/v/torch-lr-finder)](https://pypi.org/project/torch-lr-finder/)