issue #8: contributing

Author: skoro

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+# How to Contribute
+
+## Pull Requests
+
+1. Fork the `php-tkui` repository
+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 us to review and pull in new features or improvements individually.
+
+## Style Guide
+
+All pull requests must adhere to the [PSR-12 standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-12-extended-coding-style-guide.md).