Add a CONTRIBUTING.md guide

A `CONTRIBUTING` file is a file recognized by Github which is suggested upon making a new P.R.
It is quite ubiquitous among top Github projects and is way more visible than a wiki page.

This guide duplicates what is currently done in the wiki, however as recent history has
shown moving from wiki documents to Github does improve visibility and allows changes
to be reviewed (and noticed)

CONTRIBUTING.md

@@ -0,0 +1,64 @@
+# Contributing to DMD, the D programming language reference compiler
+
+First off, thanks for your interest in contributing !
+
+## Reporting bugs
+
+We exclusively use [bugzilla](https://issues.dlang.org/) for issue tracking, which is why Github issues are disabled on this repository.
+If you found a bug, please [check bugzilla](https://issues.dlang.org/query.cgi) to see if it's already reported.
+If it isn't, you can [create a new issue](https://issues.dlang.org/enter_bug.cgi).
+
+If you have question about a specific behavior, the [D.Learn](http://forum.dlang.org/group/learn) group is a good place to ask for clarification before reporting an issue.
+
+### Content
+
+When creating a new issue, make sure to include:
+- which version of DMD you are using (which can be found by running `dmd` with no argument).
+- A test case:
+  - Make it a [short, self contained and compilable example](http://sscce.org/).
+  - Avoid dependencies to foreign code (e.g. dub packages).
+  - Avoid any imports from phobos / druntime if possible.
+You can try minimizing your test case using the [DustMite tool](https://github.com/CyberShadow/DustMite/wiki).
+DustMite  is also available from our [tools](https://github.com/dlang/tools) repository and is distributed with DMD.
+
+### Regressions
+
+When finding a [regression](https://en.wikipedia.org/wiki/Software_regression), please label the issue as such:
+- Set the field 'Severity' to 'Regression' (highest level of priority)
+- Prefix the issue title with `[REG 2.XXX.Y]` where `2.XXX.Y` is the first broken version whenever possible.
+
+To help track down the point where regressions were introduced down, you can use the excellent [Digger](https://github.com/CyberShadow/digger) tool.
+Digger will automatically bisect the history for you.
+
+### Changelog
+
+We use bugzilla to list fixed issues on a new release.  This list is then included in the changelog.
+For this list to be accurate then invalid or duplicated bugs must be closed with the appropriate resolution ('RESOLVED INVALID' and 'RESOLVED DUPLICATE', respectively - as opposed to e.g. 'RESOLVED FIXED').
+
+
+## Solving bugs / Submitting pull requests
+
+Before submitting a PR there are some things you can check which will hopefully make the pulling process run smoothly.
+
+- Make sure to target the right branch.  Regressions go to stable, and everything else to master, as outlined in [our release process](http://wiki.dlang.org/DIP75).
+
+- When fixing a bugzilla issue, use the title : 'Fix issue XXXXX - Issue title'.  This is recognized by both bugzilla and our github bot (dlang-bot),
+  and will automatically link the issue and the pull request together (by providing a link to the issue in Github, and automatically closing bugs when pull requests are merged).
+
+- Document the 'why' (the change is necessary and was done this way) rather than the 'how'.
+
+- Ensure newly introduced symbols are documented and that updates to existing symbols are reflected in the documentation.
+
+- If your pull request affects the language specifications in any way (i.e. changing the grammar, deprecating a feature or adding a new one),
+  a pull request to [the website](https://github.com/dlang/dlang.org) should be submitted in parallel.
+
+- Follow the usual git good practice:
+  - Avoid changes not relevant to the issue (i.e. style issues)
+  - Separate commit for separate concerns
+  - Descriptive commit messages ease review and catching bugs
+  - Keep pull requests focused on one single topic or bug.  For example, if your fix requires a refactoring, then the refactoring should be submitted as a separate pull request.
+
+
+## dmd-internals mailing list
+
+For questions and discussions related to DMD development, a [mailing list](https://forum.dlang.org/group/dmd) is available.

README.md

@@ -7,4 +7,5 @@ To report a problem or browse the list of open bugs, please visit the
 [bug tracker](http://issues.dlang.org/).
 
 For more information, including instructions for compiling, installing, and
-hacking on DMD, visit the [D Wiki](http://wiki.dlang.org/DMD).
+hacking on DMD, check the [contribution guide](CONTRIBUTING.md) and
+visit the [D Wiki](http://wiki.dlang.org/DMD).