Document the plugin hosting process to gerrit-review
Document the plugin hosting process to gerrit-review

Put in writing what we informally do for creating new repositories
for plugins on gerrit-review.googlesource.com.

The review to the process puts clarity on the roles and responsibilities
of the plugin's author and sets a well-defined workflow for plugins
and who is responsible for maintaining and releasing them.

Change-Id: Ic322188536e732947152c1c91fdb3a9e9095ec6b

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