Add more guidelines to contributing policy

CONTRIBUTING.md

@@ -1,16 +1,15 @@
 # Contributing
 
 This module was designed for use in education; particularly for young children.
-It is not intended to replace `RPi.GPIO` and it does not claim to be suitable
-for all purposes. It is intended to provide a simple interface to everyday
-components.
+It is intended to provide a simple interface to everyday components.
 
 If a proposed change added an advanced feature but made basic usage more
 complex, it is unlikely to be added.
 
 ## Suggestions
 
-Please make suggestions by opening an
+Please make suggestions for additional components or enhancements to the
+codebase by opening an
 [issue](https://github.com/RPi-Distro/python-gpiozero/issues) explaining your
 reasoning clearly.
 
@@ -23,11 +22,22 @@ problem clearly using code examples.
 ## Documentation
 
 The documentation source lives in the
-[docs](https://github.com/RPi-Distro/python-gpiozero/tree/master/docs) folder
-and is rendered from markdown into HTML using [mkdocs](http://www.mkdocs.org/).
+[docs](https://github.com/RPi-Distro/python-gpiozero/tree/master/docs) folder.
 Contributions to the documentation are welcome but should be easy to read and
 understand.
 
+## Commit messages and pull requests
+
+Commit messages should be concise but descriptive, and in the form of a patch
+description, i.e. instructional not past tense ("Add LED example" not "Added
+LED example"). Commits that close (or intend to close) an issue should use the
+phrase "fix #123" where `#123` is the issue number.
+
+## Backwards compatibility
+
+Since this library reached v1.0 we aim to maintain backwards-compatability
+thereafter. Changes which break backwards-compatability will not be accepted.
+
 ## Python
 
 - Python 2/3 compatibility