add preamble to contributing guide
add preamble to contributing guide

we wish to make it easier to know what to expect as a contributor.
That is we want semantic versioning, and we want you to know that contributions are valuable, and that as a contributor you don't own anyone anything so there's no pressure.
domaindrivendev/swagger-rails,domaindrivendev/swagger-rails,domaindrivendev/swagger-rails