added github templates for issues and pull-requests (#2787)

fixes #2418

.github/ISSUE_TEMPLATE.md

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+
+Please make sure to **read the following list** before creating a new issue:
+
+* This issue tracker is not supposed to be used for questions on how to use visjs. It is intended to be used for bug reports and feature requests! In case you face yourself with a usage question, then post your question e.g. on [stackoverflow](https://stackoverflow.com/questions/tagged/vis.js) tagged with "vis.js".
+* Have you already used the [github search](https://github.com/almende/vis/issues), read the [documentation](http://visjs.org/) and looked at the [examples](https://github.com/almende/vis/tree/develop/examples)?
+* Make sure to mention which vis-component (network, timeline, graph2D, graph3d) you are referring to.
+* Make sure to use the [latest version of vis.js](https://cdnjs.com/libraries/vis) for bug reports.
+* Make sure to mention which browser and OS you are using when creating a bug report.
+* Please provide a minimal code example that demonstrates your issue. We recommend using [jsbin](jsbin.com) for that.
+* Delete this list from the actual issue.

.github/PULL_REQUEST_TEMPLATE.md

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+**Thank you for contributing to vis.js!!**
+
+Please make sure to check the following requirements before creating a pull request:
+
+* [ ] All pull requests must be to the [develop branch](https://github.com/almende/vis/tree/develop). Pull requests to the `master` branch will be closed!
+* [ ] Make sure your changes are based on the latest version of the [develop branch](https://github.com/almende/vis/tree/develop). (Use e.g. `git fetch && git rebase origin develop` to update you feature branch).
+* [ ] Provide an additional or update an example to demonstrate your changes or new features.
+* [ ] Update the documentation if you introduced new behavior or changed existing behavior.
+* [ ] Reference issue numbers of issues that your pull request addresses. (If you write something like `fixes #1781` in your git commit message this issue gets closed automatically by merging your pull request).
+* [ ] Expect review comments and change requests by reviewer.
+* [ ] Delete this checklist from your pull request.