Documented release steps

Author: danieldietrich

RELEASE.md

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+# Howto Release
+
+## Perform a release
+
+Release are uploaded to a staging repository of [Maven Central](https://search.maven.org):
+
+```bash
+# perform release (use `git reset --hard origin/master` and delete tags to retry)
+mvn release:clean
+mvn release:prepare
+mvn release:perform
+```
+
+Then visit [oss.sonatype.org](https://oss.sonatype.org) in order to deploy the release.
+
+It takes up to 2 hours until the release appears at [Maven Central](https://search.maven.org/#search%7Cga%7C1%7Cvavr-beanvalidation2). The public repository can be found [here](https://repo.maven.apache.org/maven2/io/vavr/vavr-beanvalidation2).
+
+## Best practice: Remove an existing release from Git
+
+Sometime something goes wrong during a release. Here are some steps to restore the previous state.
+
+Given a **<version>** (e.g. 0.9.2), a _non-published_ release can be removed like this:
+
+```bash
+# get actual master
+git clone https://github.com/vavr-io/vavr-beanvalidation2.git ; vavr-beanvalidation2
+
+# delete existing tag (local and remote)
+git tag -d v<version>
+git push --delete origin v<version>
+
+# revert development version
+mvn versions:set -DnewVersion=<version>-SNAPSHOT
+git commit -a -m "Reverted to <version>-SNAPSHOT"
+git push origin master ; git pull
+```