Adds the PR template for amundsen search service (#15)

Mostly copied from Apache Airflow, and using the same in other amundsen
repositories.

search/.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,34 @@
+Make sure you have checked **all** steps below.
+
+### Title
+
+- [ ] My PR Title addresses the issue accurately and concisely. 
+    - Example: "Updates the version of Flask to v1.0.2"
+    - In case you are adding a dependency, check if the license complies with the [ASF 3rd Party License Policy](https://www.apache.org/legal/resolved.html#category-x).
+
+### Description
+
+- [ ] Here are some details about my PR:
+
+### Tests
+
+- [ ] My PR adds the following unit tests __OR__ does not need testing for this extremely good reason:
+
+### Commits
+
+- [ ] I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "[How to write a good git commit message](http://chris.beams.io/posts/git-commit/)":
+    1. Subject is separated from body by a blank line
+    1. Subject is limited to 50 characters (not including Jira issue reference)
+    1. Subject does not end with a period
+    1. Subject uses the imperative mood ("add", not "adding")
+    1. Body wraps at 72 characters
+    1. Body explains "what" and "why", not "how"
+
+### Documentation
+
+- [ ] In case of new functionality, my PR adds documentation that describes how to use it.
+    - All the public functions and the classes in the PR contain docstrings that explain what it does
+
+### Code Quality & Coverage
+
+- [ ] Passes `make test`