allow README generation to be skipped

[bcoe]

https://github.com/googleapis/google-api-nodejs-client has a fairly bespoke README.md file, which we don't want to be repeatedly rewritten when synthtool is run on the repo. We should provide a mechanism for turning off README generation.

[JustinBeckwith]

We have that! It's the excludes property:
https://github.com/googleapis/google-api-nodejs-client/blob/master/synth.py#L27

[bcoe]

awesome, I’ll keep this open until I land a patch to exclude READMEs from the node client.

[bcoe]

addressed here: googleapis/google-api-nodejs-client#1657