Added pull request templates. (#12135)

* Added a few options for when making pull request.

* Added automatic labels and assignees.

.github/PULL_REQUEST_TEMPLATE/a--bug-fix.md

@@ -0,0 +1,19 @@
+---
+name: a) Bug fix
+about: Select this if you're fixing a bug in Keras.
+labels: bugfix
+---
+
+<!--
+Please make sure you've read and understood our contributing guidelines;
+https://github.com/keras-team/keras/blob/master/CONTRIBUTING.md
+-->
+
+**- What bug I fixed**
+
+This pull request fixes #issue_number_here .
+
+**- How I fixed it**
+
+**- How you can verify it**
+<!-- You need a good justification for not including tests for the bug you fixed. -->

.github/PULL_REQUEST_TEMPLATE/b--new-feature.md

@@ -0,0 +1,28 @@
+---
+name: b) New feature
+about: Select this if you're adding a new feature to Keras.
+labels: feature
+assignees: fchollet
+---
+
+<!--
+Please make sure you've read and understood our contributing guidelines;
+https://github.com/keras-team/keras/blob/master/CONTRIBUTING.md
+-->
+
+This pull request closes #issue_number_here .
+
+**- What I did**
+
+**- How I did it**
+
+**- How to verify it**
+<!-- 
+You need a good justification for not 
+including tests for the new feature you added. 
+-->
+
+
+- [ ] I updated the docs.
+
+This pull request adds a new feature to Keras. @fchollet, could you please take a look at it?

PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,8 @@
+<!--
+Please make sure you've read and understood our contributing guidelines;
+https://github.com/keras-team/keras/blob/master/CONTRIBUTING.md
+-->
+
 ### Summary
 
 ### Related Issues