Publish jars as a Gitea release on tag push

Add a gitea-release step to the Drone pipeline that uploads everything
in build/libs/*.jar to a Gitea Release named after the pushed tag,
authenticating with a gitea_token secret. The step is guarded with
when.event tag, and tag was added to the pipeline trigger so the build
itself runs for tag events. Document the one-time token / secret setup
and the tag workflow in the README.

.drone.yml

@@ -6,6 +6,7 @@ trigger:
   event:
     - push
     - pull_request
+    - tag
 
 steps:
   - name: gradle-build
@@ -15,3 +16,18 @@ steps:
     commands:
       - gradle --no-daemon clean build
       - ls -la build/libs
+
+  - name: gitea-release
+    image: plugins/gitea-release
+    settings:
+      api_key:
+        from_secret: gitea_token
+      base_url: https://git.nevetime.ru
+      files:
+        - build/libs/*.jar
+      title: ${DRONE_TAG}
+      checksum:
+        - sha256
+    when:
+      event:
+        - tag