Added templating guidelines to CONTRIBUTING.md
Added templating guidelines to CONTRIBUTING.md

Change-Id: I4c34434d15c963e4802fd2c815d60200e3d8ab54

GoogleCloudPlatform/magic-modules,GoogleCloudPlatform/magic-modules,GoogleCloudPlatform/magic-modules,GoogleCloudPlatform/magic-modules,GoogleCloudPlatform/magic-modules