Maven release docs (#1087)

Summary:
*Context: The Yoga OSS build has not been given a lot of attention over the past couple of years and is in serious disrepair. I'll try to get the release pipeline going again to publish to Maven Central as JCenter is shutting down. A bunch of intermediate commits may still have broken builds but will be inching closer to a working one.*

Adds some docs on how to use the release workflow.

Pull Request resolved: https://github.com/facebook/yoga/pull/1087

Reviewed By: priteshrnandgaonkar

Differential Revision: D28604686

Pulled By: passy

fbshipit-source-id: 44cac2cd0593a4f71d80df1ec5324c3c27fbe888

README.md

@@ -46,3 +46,29 @@ This will now only run the standalone webpack build upon install.
 | node           | Builds node js version.                         |
 | standalone     | Runs webpack.                                   |
 | none           | Does nothing. You can use the prepackaged libs. |
+
+## Maintainer Release Guide
+
+To publish a new release, follow these steps:
+
+1. Ensure you have your GPG key set up and your [OSS Sonatype](https://oss.sonatype.org/) credentials handy.
+2. Add the follow entries to either your local `gradle.properties` (don't forget to revert) or your global `~/.gradle/gradle.properties`:
+
+```
+# You get these from https://oss.sonatype.org/#profile;User%20Token
+mavenCentralRepositoryUsername=<username>
+mavenCentralRepositoryPassword=<password>
+
+# You can get the keyId (in GPG 1.4 format) by running `gpg1 --list-keys`.
+signing.secretKeyRingFile=</path/to/secring.gpg>
+signing.keyId=<key_id>
+signing.password=<key_password>
+```
+
+3. Change the `VERSION_NAME` in `gradle.properties` to a non-SNAPSHOT release.
+4. Commit and land the version change.
+5. Run `./gradlew publishToMaven`.
+6. Run `./gradlew closeAndReleaseRepository`.
+7. Change the `VERSION_NAME` in `gradle.properties` back to a new SNAPSHOT release.
+8. Commit and land the version change.
+9. Celebrate! You've made a release!