Update opentelemetry-proto README with instructions

[aabmass]

Add instructions on how to regenerate/maintain the generated proto files to the README file

[aabmass]

Can someone assign to me? :)

[codeboten]

Done!

[ocelotl]

BTW, I was a bit confused about the VIRTUAL_ENV variable used in the proto-generating script. It could be something to explain in the README as well.