Add pull request template (#2326)

.github/pull_request_template.md

@@ -0,0 +1,31 @@
+<!--
+    Thank you for your contribution!
+    Please see the contributing guide: https://github.com/google/.github/blob/master/CONTRIBUTING.md
+
+    Keep in mind that Gson is in maintenance mode. If you want to add a new feature, please first search for existing GitHub issues, or create a new one to discuss the feature and get feedback.
+-->
+
+### Purpose
+<!-- Describe the purpose of this pull request, for example which new feature it adds or which bug it fixes -->
+<!-- If this pull request closes a GitHub issue, please write "Closes #<issue>", for example "Closes #123" -->
+
+
+### Description
+<!-- If necessary provide more information, for example relevant implementation details or corner cases which are not covered yet -->
+<!-- If there are related issues or pull requests, link to them by referencing their number, for example "pull request #123" -->
+
+
+
+### Checklist
+<!-- The following checklist is mainly intended for yourself to verify that you did not miss anything -->
+
+- [ ] New code follows the [Google Java Style Guide](https://google.github.io/styleguide/javaguide.html)
+- [ ] If necessary, new public API validates arguments, for example rejects `null`
+- [ ] New public API has Javadoc
+    - [ ] Javadoc uses `@since $next-version$`  
+      (`$next-version$` is a special placeholder which is automatically replaced during release)
+- [ ] If necessary, new unit tests have been added  
+  - [ ] Assertions in unit tests use [Truth](https://truth.dev/), see existing tests
+  - [ ] No JUnit 3 features are used (such as extending class `TestCase`)
+  - [ ] If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
+- [ ] `mvn clean verify javadoc:jar` passes without errors

README.md

@@ -69,6 +69,22 @@ Please use the ['gson' tag on StackOverflow](https://stackoverflow.com/questions
   * [Gson Tutorial Series](https://futurestud.io/tutorials/gson-getting-started-with-java-json-serialization-deserialization) by `Future Studio`
   * [Gson API Report](https://abi-laboratory.pro/java/tracker/timeline/gson/)
 
+### Building
+
+Gson uses Maven to build the project:
+```
+mvn clean verify
+```
+
+JDK 11 or newer is required for building, JDK 17 is recommended.
+
+### Contributing
+
+See the [contributing guide](https://github.com/google/.github/blob/master/CONTRIBUTING.md).  
+Please perform a quick search to check if there are already existing issues or pull requests related to your contribution.
+
+Keep in mind that Gson is in maintenance mode. If you want to add a new feature, please first search for existing GitHub issues, or create a new one to discuss the feature and get feedback.
+
 ### License
 
 Gson is released under the [Apache 2.0 license](LICENSE).