add publish snapshot action (#25)

.github/workflows/maven_build.yml

 name: Maven Build
 
 on:
-  push:
-    branches:
-      - master
   pull_request:
     branches:
       - master
@@ -20,4 +17,4 @@ jobs:
           java-version: '8'
           distribution: 'zulu'
       - name: Build with Maven
-        run: mvn clean package
+        run: mvn clean install

.github/workflows/maven_publish_snapshot.yml

+name: Maven Publish Snapshot
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  publish-snapshot:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup Apache Maven Central
+        uses: actions/setup-java@v2
+        with:
+          java-version: '8'
+          distribution: 'zulu'
+          server-id: sonatype-nexus-snapshots
+          server-username: MAVEN_USERNAME
+          server-password: MAVEN_PASSWORD
+          gpg-passphrase: GPG_PASSPHRASE
+          gpg-private-key: ${{ secrets.GPG_PRIVATE_KEY }}
+      - name: Publish to Apache Maven Central
+        run: mvn clean verify gpg:sign install:install deploy:deploy
+        env:
+          MAVEN_USERNAME: ${{ secrets.OSSRH_USERNAME }}
+          MAVEN_PASSWORD: ${{ secrets.OSSRH_PASSWORD }}
+          GPG_PASSPHRASE: ${{ secrets.GPG_PASSPHRASE }}