docs: add publishing documentation

PUBLISHING.md

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+# Publishing a release
+
+This document describes the process of publishing a new version of the package.
+
+1. Update the version in the `composer.json` file.
+2. Commit the changes.
+	-	```bash
+		git commit -m "chore: bump version to <version>"
+		```
+
+		Where `<version>` is the new version number (e.g. `1.2.3`).
+3. Create a new tag.
+	-	```bash
+		git tag v<version>
+		```
+
+		Where `<version>` is the new version number (e.g. `1.2.3`).
+4. Push the changes.
+	-	```bash
+		git push origin master --tags
+		```
+
+A release will automatically be created.