Include README and adjust action metadata

Action needed to be renamed before publishing to GitHub Marketplace, and a README needed to be written. The README contains all available inputs and outputs, as well as an example on how to use the action.

README.md  | 41 +++++++++++++++++++++++++++++++++++++++++
action.yml |  5 ++++-
2 files changed, 45 insertions(+), 1 deletion(-)

Author: matootie

README.md

@@ -0,0 +1,41 @@
+# GitHub Docker Action
+
+Build and publish your repository as a Docker image and push it to GitHub Package Registry in one easy step.
+
+## Inputs
+
+### `username`
+
+**Required**. GitHub user to publish the image on behalf of.
+
+### `personalAccessToken`
+
+**Required**. GitHub Personal Access Token for the user. Must have write permissions for packages.
+
+### `repositoryName`
+
+Optional. The repository to push the image to. Defaults to current repository. Must be specified in format `user/repo`.
+
+### `imageName`
+
+Optional. The desired name for the image. Defaults to current repository name.
+
+### `imageTag`
+
+Optional. The desired tag for the image. Defaults to current branch or release version number.
+
+## Outputs
+
+### `imageURL`
+
+The full URL of the image.
+
+## Example usage
+
+```yaml
+- name: Publish Image
+  uses: matootie/github-docker@v1.0.0
+  with:
+    username: matootie
+    personalAccessToken: ${{ secrets.PERSONAL_ACCESS_TOKEN }}
+```

action.yml

@@ -1,4 +1,4 @@
-name: 'GitHub Docker'
+name: 'GitHub Docker Action'
 description: 'Build and publish your repository as a Docker image and push it to GitHub Package Registry in one step'
 inputs:
   username:
@@ -22,3 +22,6 @@ outputs:
 runs:
   using: 'node12'
   main: 'dist/index.js'
+branding:
+  icon: 'anchor'
+  color: 'blue'