Add a note about other annotation processors to the maven example
Add a note about other annotation processors to the maven example

Fixes #1187

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=311831016

cushon/error-prone,cushon/error-prone,cushon/error-prone,google/error-prone,google/error-prone,cushon/error-prone