Docs for creating a release

Signed-off-by: Andrew DeMaria <lostonamountain@gmail.com>

contrib/release.md

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+Release Steps
+=============
+
+1. Ensure changelog is up to date
+
+2. Create a new minor branch if not already exists. Checkout branch
+
+        git checkout -b release-X.Y
+
+3. Bump the maven pom
+
+        mvn versions:set -DnewVersion=X.Y.Z-RELEASE
+
+4. Commit maven pom changes
+
+
+5. Create a new tag
+
+        git tag -s vX.Y.Z -m 'Release vX.Y.Z' 
+
+6. Package
+
+        mvn clean verify -P docker,sign
+
+7. push up branch and tag
+
+        git push origin vX.Y.Z
+        git push -u origin release-X.Y
+
+8. Create new release on github
+
+   - Draft new Relase
+   - Choose existing tag
+   - Title is "Airsonic X.Y.Z"
+   - Contents are the relevant entry of the CHANGELOG.md file
+   - Upload `airsonic.war` and `airsonic.war.asc`
+
+9. Update latest docker tag
+
+        docker tag airsonic/airsonic:X.Y.Z-RELEASE airsonic/airsonic:latest
+
+10. Docker login with airsonic credentials in `airsonic-passwords` repo
+
+        docker login
+
+11. Push images
+
+        docker push airsonic/airsonic:X.Y.Z-RELEASE
+        docker push airsonic/airsonic:latest
+
+12. Checkout master branch and bump maven version to next snapshot version
+
+        git checkout master
+        mvn versions:set -DnewVersion=X.Y+1.0-SNAPSHOT
+
+13. Git commit and push