Expand documentation on contributing new resources

[iyabchen]

#245

• mentioned compile_common_files and copy_common_files
• added IAM handwritten file need to add merge functions and fetch functions
• added adding test file example steps

[melinath]

LGTM - definitely an improvement!

[melinath]

I think this might actually be a little out of date - specifically, the part about Add your new test cases to test/cli_test.go... - nowadays I think they get added to https://github.com/GoogleCloudPlatform/magic-modules/tree/master/mmv1/third_party/validator/tests/data, and the compilation code automatically detects them and adds them to the lists in cli_test and read_test.

I think it's out of scope for this change - but we can also generate tests automatically these days, though it may require changing the test generation allowlist. Some details on this in b/211888856#comment2.

[melinath]

LGTM