[*]: add "CONTRIBUTING.md"

.gitattributes

@@ -12,5 +12,6 @@
 /circle.yml export-ignore
 /phpdox.xml export-ignore
 /phpunit.xml export-ignore
+/CONTRIBUTING.md export-ignore
 /ISSUE_TEMPLATE.md export-ignore
 /PULL_REQUEST_TEMPLATE.md export-ignore

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+# How to Contribute
+
+## Pull Requests
+
+1. Create your own [fork](https://help.github.com/articles/fork-a-repo) of this repo
+2. Create a new branch for each feature or improvement
+3. Send a pull request from each feature branch to the **master** branch
+
+It is very important to separate new features or improvements into separate
+feature branches, and to send a pull request for each branch. This allows me to
+review and pull in new features or improvements individually.
+
+## Style Guide
+
+All pull requests must adhere to the [PSR-2 standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md).
+
+## Unit Testing
+
+All pull requests must be accompanied by passing PHPUnit unit tests and
+complete code coverage.
+
+[Learn about PHPUnit](https://github.com/sebastianbergmann/phpunit/)