Added PULL_REQUEST_TEMPLATE.md

Change-Id: Iad9c09434f87219e384b294d6221f4cac7a839eb

.github/PULL_REQUEST_TEMPLATE.md

+Thank you for submitting a PR!
+
+Please make sure you include the names of the affected language(s) in your PR title.
+This helps us get the correct maintainers to look at your issue.
+
+If you make changes to any of the code generators, be sure to run
+`cd tests && sh generate_code.sh` (or equivalent .bat) and include the generated
+code changes in the PR. This allows us to better see the effect of the PR.
+
+If your PR includes C++ code, please adhere to the Google C++ Style Guide,
+and don't forget we try to support older compilers (e.g. VS2010, GCC 4.6.3),
+so only some C++11 support is available.
+
+Include other details as appropriate.
+
+Thanks!