Update CONTRIBUTING.md

Author: mevdschee

CONTRIBUTING.md

@@ -1,3 +1,18 @@
 # Contributing to php-crud-api
 
-Pull requests are welcome. Please use "phpfmt" and run the build and the tests.
+Pull requests are welcome.
+
+## Use phpfmt
+
+Please use "phpfmt" to ensure consistent formatting.
+
+## Run the tests
+
+Before you do a PR, you should ensure any new functionality has test cases and that all existing tests are succeeding.
+
+## Run the build 
+
+Since this project is a single file application, you must ensure that classes are loaded in the correct order. 
+This is only important for the "extends" and "implements" relations. The build script appends the classes in 
+alphabetical order (directories first). The path of the class that is extended or implemented must be alphabetically
+first. The build will fail with a "Class not found" error message if you get this wrong.