Add a documentation for release process (#367)

RELEASING.md

@@ -0,0 +1,76 @@
+# Release process
+We release protobuf-gradle-plugin to both Maven Central and Gradle Plugin Portal.
+
+Prerequisites
+--------------
+1. If you haven't deployed artifacts to Maven Central or Gradle Plugin Portal before, you need to
+set up your OSSRH (OSS Repository Hosting) account and Gradle Plugin Portal account.
+
+2. Make sure your account is added to the project on Sonatype and Gradle Plugin Portal.
+
+- Sonatype: create a support ticket on [issues.sonatype.org](issues.sonatype.org) to acquire access 
+to `com.google.protobuf:protobuf-gradle-plugin` project.
+
+- Gradle Plugin Portal: follow instructions on [this page](https://plugins.gradle.org/docs/reclaiming)
+to acquire permission for publishing new versions.
+
+3. Generate and publish GnuPG key
+- Run `gpg --full-generate-key` to generate the key.
+- Run `gpg -keyring secring.gpg --export-secretkeys > ~/.gnupg/secring.gpg` to export your key.
+- Run `gpg --keyserver certserver.pgp.com --send-keys <keyId>` to publish your key, where `keyId`
+is teh last 8 characters of the long hex from `gpg --list=keys`. You may need to prefix `keyId`
+with `0x` notation.
+
+4. Edit your Gradle user properties file (default location is `$USER_HOME/.gradle/gradle.properties`)
+- Make sure `ossrhUsername` and `ossrhPassword` are added according to your S
+onatype user credentials.
+- Make sure `gradle.publish.key` and `gradle.publish.secret` are added according to your API key
+under the "API Keys" tab in your profile page of Gradle Plugin Portal.
+- Add your key information:
+```
+signing.keyId=<last 8 characters of the long hex from gpg --list-keys>
+signing.password=<your password for signing your GPG key>
+signing.secretKeyRingFile=</path/to/.gnupg/secring.gpg>
+```
+
+Releasing
+----------
+1. Make release commit
+- Edit `build.gradle`: 
+  - remove “-SNAPSHOT” from `version`. Assuming the version is `$RELEASE_VERSION`.
+- Edit `README.md`:
+  - The “latest version” shown should be a version prior to `$RELEASE_VERSION`. Will refer it 
+  as `$PREV_VERSION`
+  - Replace all `$PREV_VERSION` to `$RELEASE_VERSION`
+  - Update Gradle and/or Java version requirement if necessary
+- Run `./gradlew clean build`
+- Run `git commit -a -m “$RELEASE_VERSION release”`
+- Run `git tag -a v$RELEASE_VERSION -m “The $RELEASE_VERSION release”`
+
+2. Make commit for next version
+- Refer to the next version as `$NEXT_VERSION`
+- Edit `build.gradle`: change version from `$RELEASE_VERSION` to `$NEXT_VERSION-SNAPSHOT`.
+- Edit `README.md`: replace `$RELEASE_VERSION-SNAPSHOT` with `$NEXT_VERSION-SNAPSHOT`.
+- Run `git commit -a -m “Start $NEXT_VERSION development cycle”`
+
+3. Publish artifacts
+- Run `git checkout v$RELEASE_VERSION`
+- Release on Maven Central
+  - Run `./gradlew uploadArchives`
+  - Go to the [OSSRH site](https://oss.sonatype.org), under “Staging Repositories”, close and release the 
+  artifact.
+- Release on Gradle Plugin Portal
+  - Run `./gradlew publishPlugins`
+- Verify that artifacts are available on [Maven Central](https://search.maven.org/artifact/com.google.protobuf/protobuf-gradle-plugin) 
+(may take a few minutes up to hours) and [Gradle Plugin Portal](https://plugins.gradle.org/plugin/com.google.protobuf) 
+(should be available immediately).
+
+4. Push commits
+- Run `git push upstream master`
+- Run `git push --tags upstream`
+
+5. Create release notes
+
+
+
+