Add PUBLISHING.md

Author: malczuuu

PUBLISHING.md

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+# Publishing
+
+Keep Git tags with `vX.Y.Z-suffix` format. GitHub Actions job will only trigger on such tags and will remove `v` prefix.
+
+See [`gradle-release.yml`](.github/workflows/gradle-release.yml) for whole publishing procedure.
+
+Set the following environment variables in your CI/CD (GitHub Actions, etc.):
+
+```txt
+# generated on Sonatype account
+PUBLISHING_USERNAME=<username>
+PUBLISHING_PASSWORD=<password>
+
+# generated PGP key for signing artifacts
+SIGNING_KEY=<PGP key>
+SIGNING_PASSWORD=<PGP password>
+```
+
+Artifacts are published to Maven Central via Sonatype, using following Gradle task.
+
+```bash
+./gradlew -Pversion=<version> -Psign publishAggregationToCentralPortal
+```
+
+This command uses `nmcp` Gradle plugin - [link](https://github.com/GradleUp/nmcp).
+
+**Note** that this only uploads the artifacts to Sonatype repository. You need to manually log in to Sonatype to push
+the artifacts to Maven Central.