Link to contributing guidelines, add some hints

It makes development docs more visible from the repository, it follows
best practices for Open Source Maintainers on GitHub. It initiates
moving contributing guidelines from the wiki to the repository itself.

CONTRIBUTING.md

@@ -0,0 +1,72 @@
+# Contributing to MusicBrainz Server
+
+Thank you for considering contributing to MusicBrainz Server.
+
+Get started with <https://musicbrainz.org/doc/Development> first,
+then continue reading the additional guidelines below.
+
+Some more general guidelines (not complete yet) are available at
+<https://github.com/metabrainz/guidelines/> too.
+
+## Submitting changes
+
+### Commit message
+
+Please follow the generally accepted [seven rules of a great Git
+commit message](https://chris.beams.io/posts/git-commit/#seven-rules):
+
+1. Separate subject from body with a blank line
+2. Limit the subject line to 50 characters
+3. Capitalize the subject line
+4. Do not end the subject line with a period
+5. Use the imperative mood in the subject line
+6. Wrap the body at 72 characters
+7. Use the body to explain _what_ and _why_ vs. _how_
+
+Additionally, start the subject line with a ticket reference if applicable.
+
+### Pull request
+
+#### Ticket
+
+If your change is large or relevant to users, it should have a ticket
+in [our issue tracker](https://tickets.metabrainz.org/browse/MBS).
+Create one if necessary.
+Reference it with its key `MBS-XXX`.
+It will be used to follow the progress of the change and to generate
+the release notes that are made available to users on the blog.
+
+Untracked changes are typos, comments, coding style changes, automated
+dependency updates, unnoticeable refactoring, and so on.
+
+#### Title
+
+Describe your change with a short imperative title, e.g.
+
+> Change small unnoticeable bits
+
+If your change resolves a ticket (see [above](#ticket)), please make sure you
+prefix your pull request title with `MBS-XXX: ` in order for our issue tracker
+to link your pull request to that ticket, e.g.
+
+> MBS-1234567: Change things relevant to users
+
+If it **partially resolves** a ticket, use parenthesis, e.g.
+
+> MBS-1234567 (I): Make first part of needed changes
+
+If your change relate to **several tickets**, separate these with commas, e.g.
+
+> MBS-1234567, MBS-2345678: Change two related things at once
+
+#### Comment
+
+Just follow our [pull request template](.github/PULL_REQUEST_TEMPLATE.md).
+
+If your change relates to a ticket, make sure to mention it in the comment, e.g.
+
+```Markdown
+# Summary
+
+Fix MBS-1234567: Change things relevant to users
+```

README.md

@@ -31,12 +31,13 @@ Contributing
 ------------
 
 Please submit all patches to [GitHub](https://github.com/metabrainz/musicbrainz-server/pulls) for review.
+See the [contributing guidelines](CONTRIBUTING.md).
 
 License
 -------
 
 MusicBrainz Server is released under the GPLv2 or later.
-See the [licenses notice](COPYING.md).
+See the [license notice](COPYING.md).
 
 Further reading
 ---------------