[docs] Split release docs into two files

Releases-publish.md

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+## Publish a release (same steps for an rc version, 0.x.0)
+
+Publish to Maven Central (Note: **We could get rid of this whole section by publishing binaries to npm instead**):
+  - Log into Sonatype and go to [Staging upload](https://oss.sonatype.org/#staging-upload). You'll need to get permissions for this by filing a ticket explaining you're a core contributor to React Native. [Example ticket](https://issues.sonatype.org/browse/OSSRH-11885).
+  - Select Artifact(s) with a POM (to publish to a local Maven repo for testing run `./gradlew :ReactAndroid:installArchives`)
+  - Add all files: .aar, sources jar, javadoc jar, .asc for everything (including the POM file)
+  - Wait a few hours until you see the version has propagated to [JCenter](https://bintray.com/bintray/jcenter/com.facebook.react%3Areact-native/view)
+
+To release to npm:
+
+(You need to be a maintainer of the repo. For admins, here's the command to promote someone)
+
+```
+npm owner add <user> react-native
+```
+
+```
+git tag v0.version_you_are_releasing.0-rc 0.version_you_are_releasing-stable # don't forget the `v` at the beginning!
+git push --tags
+```
+
+- Publish to npm
+
+```
+npm set registry https://registry.npmjs.org/
+npm publish
+# *Only* when doing a non-rc release:
+# npm dist-tag add [email protected]_rc_version.0 latest
+```
+- Upgrade tags to a release by going to https://github.com/facebook/react-native/tags
+- Click "Add Notes to release"
+- Click Publish