Create CONTRIBUTING.md (#1278)

According to https://help.github.com/articles/setting-guidelines-for-repository-contributors, a link to a file named CONTRIBUTING.md will be shown when someone is about to create a pull request. That is more visible than our Wiki pages, which are easy to miss.

Create the CONTRIBUTING.md file, based on the existing Wiki page https://github.com/erlang/otp/wiki/Contribution-Guidelines.

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+# Contributing to Erlang/OTP
+
+## Reporting a bug
+
+Report bugs at https://bugs.erlang.org. See [Bug reports](https://github.com/erlang/otp/wiki/Bug-reports)
+for more information.
+
+## Submitting Pull Requests
+
+You can contribute to Erlang/OTP by opening a Pull Request.
+
+## Fixing a bug
+
+* In most cases, pull requests for bug fixes should be based on the `maint` branch.
+There are exceptions, for example corrections to bugs that have been introduced in the `master` branch.
+
+* Include a test case to ensure that the bug is fixed **and that it stays fixed**.
+
+* TIP: Write the test case **before** fixing the bug so that you can know that it catches the bug.
+
+* For applications without a test suite in the git repository, it would be appreciated if you provide a
+small code sample in the commit message or email a module that will provoke the failure.
+
+## Adding a new feature
+
+* In most cases, pull requests for new features should be based on the `master` branch.
+
+* It is recommended to discuss new features on
+[the erlang-questions mailing list](http://erlang.org/mailman/listinfo/erlang-questions),
+especially for major new features or any new features in ERTS, Kernel, or STDLIB.
+
+* It is important to write a good commit message explaining **why** the feature is needed.
+We prefer that the information is in the commit message, so that anyone that want to know 
+two years later why a particular feature can easily find out. It does no harm to provide
+the same information in the pull request (if the pull request consists of a single commit,
+the commit message will be added to the pull request automatically).
+
+* With few exceptions, it is mandatory to write a new test case that tests the feature.
+The test case is needed to ensure that the features does not stop working in the future.
+
+* Update the [Documentation](https://github.com/erlang/otp/wiki/Documentation) to describe the feature.
+
+* Make sure that the new feature builds and works on all major platforms. Exceptions are features
+that only makes sense one some platforms, for example the `win32reg` module for accessing the Windows registry.
+
+* Make sure that your feature does not break backward compatibility. In general, we only break backward
+compatibility in major releases and only for a very good reason. Usually we first deprecate the
+feature one or two releases beforehand.
+
+* In general, language changes/extensions require an
+[EEP (Erlang Enhancement Proposal)](https://github.com/erlang/eep) to be written and approved before they 
+can be included in OTP. Major changes or new features in ERTS, Kernel, or STDLIB will need an EEP or at least
+a discussion on the mailing list.
+
+## Before you submit your pull request
+
+* Make sure existing test cases don't fail. It is not necessary to run all tests (that would take many hours),
+but you should at least run the tests for the application you have changed.
+See [Running tests](https://github.com/erlang/otp/wiki/Running-tests).
+
+Make sure that your branch contains clean commits:
+
+* Don't make the first line in the commit message longer than 72 characters.
+**Don't end the first line with a period.**
+
+* Follow the guidelines for [Writing good commit messages](https://github.com/erlang/otp/wiki/Writing-good-commit-messages).
+
+* Don't merge `maint` or `master` into your branch. Use `git rebase` if you need to resolve merge
+conflicts or include the latest changes.
+
+* To make it possible to use the powerful `git bisect` command, make sure that each commit can be
+compiled and that it works.
+
+* Check for unnecessary whitespace before committing with `git diff --check`.
+
+Check your coding style:
+
+* Make sure your changes follow the coding and indentation style of the code surrounding your changes.
+
+* Do not commit commented-out code or files that are no longer needed. Remove the code or the files.
+
+* In most code (Erlang and C), indentation is 4 steps. Indentation using only spaces is **strongly recommended**.
+
+### Configuring Emacs
+
+If you use Emacs, use the Erlang mode, and add the following lines to `.emacs`:
+
+    (setq-default indent-tabs-mode nil)
+    (setq c-basic-offset 4)
+
+If you want to change the setting only for the Erlang mode, you can use a hook like this:
+
+```
+(add-hook 'erlang-mode-hook 'my-erlang-hook)
+
+(defun my-erlang-hook ()
+  (setq indent-tabs-mode nil))
+```