Document caching

Author: Greg Bowler

README.md

@@ -23,10 +23,63 @@ jobs:
 
     steps:
     - uses: actions/checkout@v1
-    - uses: phpactions/composer@master
+    - uses: php-actions/composer@v1
     # ... then your own project steps ...
 ```
 
+Running custom commands
+-----------------------
+
+By default, adding `- uses: php-actions/composer@v1` into your workflow will run `composer install`, as `install` is the default command name.
+
+You can issue custom commands by passing a `command` input, like so:
+
+```yaml
+...
+
+jobs:
+  build:
+
+    ...
+
+    - name: Install dependencies
+      uses: php-actions/composer@v1
+      with:
+        command: your-command-here
+```
+
+Caching dependencies for faster builds
+--------------------------------------
+
+Github actions supports dependency caching, allowing the `vendor/` directory contents to be cached between workflows, as long as the `composer.lock` file has not changed. This produces much faster builds, as the `composer install` command does not have to be run at all if the cache is valid.
+
+Example workflow (taken from https://github.com/PhpGt/Dom):
+
+```yaml
+name: CI
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: [ubuntu-latest]
+    
+    steps:
+    - uses: actions/checkout@v1
+      
+    - name: Cache PHP dependencies
+      uses: actions/cache@v1
+      with:
+        path: vendor
+        key: ${{ runner.OS }}-build-${{ hashFiles('**/composer.lock') }}
+          
+    - uses: php-actions/composer@master
+
+    ...      
+```
+
+In the example above, the "key" is passed to the Cache action that consists of a hash of the composer.lock file. This means that as long as the contents of composer.lock doesn't change between workflows, the vendor directory will be persisted between workflows.
+
 [php-actions-phpunit]: https://github.com/marketplace/actions/phpunit-php-actions 
 [php-actions-phpspec]: https://github.com/marketplace/actions/phpspec-php-actions 
-[php-actions-behat]: https://github.com/marketplace/actions/behat-php-actions 
+[php-actions-behat]: https://github.com/marketplace/actions/behat-php-actions