Summary:
Update docs to prevent build process from failing when collaborators do not have MYAPP_RELEASE_STORE_FILE in their env.
Build process often fails when following the existing documentation on setting the signingConfigs in build.gradle, as not all collaborators have the required ENV set in their gradle.properties
Closes https://github.com/facebook/react-native/pull/11716
Differential Revision: D4389150
Pulled By: hramos
fbshipit-source-id: 37941382950fac41f19d19317f49769588b135b4