Move contributing guide to its own file

This file will be linked to when submitting a pull request on Github.
Also, Github will notify contributors when it changes. There might be
more features I do not know about.

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# How to Contribute
+
+Just follow 3 rules:
+
+- **1 feature per pull-request**
+- **New features need tests**
+- Tests, coding standards and PHPStan **checks must pass**:
+
+    ```bash
+    composer complete-check
+    ```
+
+    Do you need to fix coding standards? Run:
+
+    ```bash
+    composer fix-cs
+    ```
+
+We would be happy to accept PRs that follow these guidelines.

README.md

@@ -442,23 +442,7 @@ Use [the `create` command](/docs/RectorRecipe.md).
 
 ## How to Contribute
 
-Just follow 3 rules:
-
-- **1 feature per pull-request**
-- **New features need tests**
-- Tests, coding standards and PHPStan **checks must pass**:
-
-    ```bash
-    composer complete-check
-    ```
-
-    Do you need to fix coding standards? Run:
-
-    ```bash
-    composer fix-cs
-    ```
-
-We would be happy to accept PRs that follow these guidelines.
+See [the contribution guide](/CONTRIBUTING.md).
 
 ## Run Rector in Docker
 You can run Rector on your project using Docker: