Added PR template

Author: aosingh

.github/pull_request_template.md

@@ -0,0 +1,21 @@
+Thanks for contributing!
+
+Before submitting PRs for dbt-oracle you must have your signed *Oracle
+Contributor Agreement* accepted.  See
+https://www.oracle.com/technetwork/community/oca-486395.html
+
+If the problem solved is small, you may find it easier to open an Issue
+describing the problem and its cause so we can create the fix.
+
+The bottom of your commit message must have the following line using your name
+and e-mail address as it appears in the OCA Signatories list.
+
+```
+Signed-off-by: Your Name <you@example.org>
+```
+
+This can be automatically added to pull requests by committing with:
+
+```
+git commit --signoff
+````