Update issue templates

.github/ISSUE_TEMPLATE/a--tensorflow-backend-users.md

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+---
+name: a) TensorFlow backend users
+about: Select this is you're using Keras with the TensorFlow backend (default).
+
+---
+
+Please make sure that the boxes below are checked before you submit your issue.
+If your issue is an **implementation question**, please ask your question on [StackOverflow](http://stackoverflow.com/questions/tagged/keras) or [on the Keras Slack channel](https://keras-slack-autojoin.herokuapp.com/) instead of opening a GitHub issue.
+
+Thank you!
+
+- [ ] Check that you are up-to-date with the master branch of Keras. You can update with:
+`pip install git+git://github.com/keras-team/keras.git --upgrade --no-deps`
+
+- [ ] Check that your version of TensorFlow is up-to-date. The installation instructions can be found [here](https://www.tensorflow.org/get_started/os_setup).
+
+- [ ] Provide a link to a GitHub Gist of a Python script that can reproduce your issue (or just copy the script here if it is short).

.github/ISSUE_TEMPLATE/b--theano-backend-users.md

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+---
+name: b) Theano backend users
+about: Select this if you're using Keras with the Theano backend.
+
+---
+
+Please make sure that the boxes below are checked before you submit your issue.
+If your issue is an **implementation question**, please ask your question on [StackOverflow](http://stackoverflow.com/questions/tagged/keras) or [on the Keras Slack channel](https://keras-slack-autojoin.herokuapp.com/) instead of opening a GitHub issue.
+
+Thank you!
+
+- [ ] Check that you are up-to-date with the master branch of Keras. You can update with:
+`pip install git+git://github.com/keras-team/keras.git --upgrade --no-deps`
+
+- [ ] Check that you are up-to-date with the master branch of Theano. You can update with:
+`pip install git+git://github.com/Theano/Theano.git --upgrade --no-deps`
+
+- [ ] Provide a link to a GitHub Gist of a Python script that can reproduce your issue (or just copy the script here if it is short).

.github/ISSUE_TEMPLATE/c--cntk-backend-users.md

@@ -0,0 +1,17 @@
+---
+name: c) CNTK backend users
+about: Select this if you're using Keras with the CNTK backend.
+
+---
+
+Please make sure that the boxes below are checked before you submit your issue.
+If your issue is an **implementation question**, please ask your question on [StackOverflow](http://stackoverflow.com/questions/tagged/keras) or [on the Keras Slack channel](https://keras-slack-autojoin.herokuapp.com/) instead of opening a GitHub issue.
+
+Thank you!
+
+- [ ] Check that you are up-to-date with the master branch of Keras. You can update with:
+`pip install git+git://github.com/keras-team/keras.git --upgrade --no-deps`
+
+- [ ] Check that your version of CNTK is up-to-date.
+
+- [ ] Provide a link to a GitHub Gist of a Python script that can reproduce your issue (or just copy the script here if it is short).