contributing: Add some basic contribution guidelines

Signed-off-by: Paul Barker <[email protected]>

CONTRIBUTING.md

@@ -0,0 +1,11 @@
+# Contributing to meta-linux-mainline
+
+Please review the [Goals and non-goals of this layer][1] in the project README
+before opening issues or submitting pull requests.
+
+[1]: README.md#goals-and-non-goals-of-this-layer
+
+Please also do not submit pull requests to update the linux-stable or
+linux-mainline recipes to new kernel releases - it is more effort for me as
+maintainer to review such pull requests than it is to run the update-layer
+script myself.