feat(init): Add action.yml metadata & readme

This should be the only commit, unless I messed something up, or the github
context changes in the future.

Adds both the action.yml file and the readme.

Readme.md

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+# Set Git Branch
+
+This is a dead-simple composite action that uses the github context to provide
+an environment variable (`$GIT_BRANCH`) and an output (`branch-name`) with the
+branch name that workflow creators are most likely to care about.
+
+## Pull Request Events
+
+For Pull Request events (`pull_request` & `pull_request_target`) the `github`
+context provides the `github.head_ref` context variable that contains the name
+of the source branch for the PR.
+
+## Other `branch`-type Events
+
+For other events triggered by branches, the [`github`
+context](https://docs.github.com/en/actions/learn-github-actions/contexts#github-context)
+provides the `github.ref_name` variable with the name of the branch or tag that
+triggered the event, and this is what is used to set the env var & output
+
+## Tag Events
+
+Whenever `github.ref_type` is set to `tag`, the current event was triggered by a
+tag, and the `github` context does not provide any branch-relevant information,
+so this action provides an empty output and environment variable.

action.yml

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+name: 'Set Git Branch'
+author: Joshua Estrin Skrzypek <https://github.com/jskrzypek>
+description: |
+  Sets the git branch name as a consistent env var & output. If the event is 
+  triggered by a tag, no value will be set.
+branding:
+  icon: git-branch
+  color: orange
+outputs:
+  branch-name:
+    description: "The branch name"
+    value: ${{ env.GIT_BRANCH }}
+runs:
+  using: "composite"
+  steps:
+    - name: Set Git branch name on PR
+      id: branch-name-from-pr
+      if: |-
+        github.event_name == 'pull_request'
+        || github.event_name == 'pull_request_target'
+      run: |
+        echo "GIT_BRANCH=${{ github.head_ref }}" >> $GITHUB_ENV
+        echo "::set-output name=branch-name::${{ github.head_ref }}"
+      shell: bash
+    - name: Set Env Vars on non-PR branch events
+      id: branch-name-from-non-pr
+      if: |-
+        github.event_name != 'pull_request'
+        && github.event_name != 'pull_request_target'
+        && github.ref_type == 'branch'
+      run: |
+        echo "GIT_BRANCH=${{ github.ref_name }}" >> $GITHUB_ENV
+        echo "::set-output name=branch-name::${{ github.head_ref }}"
+      shell: bash