add PR template

.github/pull_request_template.md

+
+
+Thanks for your contribution!
+
+Unless you want to send a simple several lines of PR that can be easily merged, please note the following:
+
+* If you want to add a new feature, please open an issue first and indicate that you want to
+	contribute.
+
+	There are features that we prefer to not add to tensorpack, e.g. symbolic models
+  (see details at https://tensorpack.readthedocs.io/tutorial/symbolic.html).
+	Therefore it's good to have a discussion first.
+
+* If you want to add a new example, please note that:
+
+	1. We prefer to not have an example that is too similar to existing ones in terms of the tasks.
+
+	2. Examples have to be able to reproduce (preferrably in some mesurable metrics) published or well-known experiments and results.
+
+* Please run `flake8 .` under the root of this repo to lint your code, and make sure the command produces no output.