Adds squashaton rules closes #1987

Author: JJ

CONTRIBUTING.md

@@ -42,6 +42,7 @@ in the [#perl6 IRC channel](https://perl6.org/community/irc).
 * Be explicit about routine signatures. If a method accepts a `*%args`,
   but treats some of them specially, list them separately.
 * Check out [the styleguide](writing-docs/STYLEGUIDE.md) for further guidance.
+* If you are participating in one of the montly squashathons, [please read this guide](writing-docs/SQUASHATHONS.md).
 
 ## Writing Code Examples
 

writing-docs/SQUASHATONS.md

@@ -0,0 +1,9 @@
+# Squashathon HOWTO
+
+[SQUASHathons](https://github.com/rakudo/rakudo/wiki/Monthly-Bug-Squash-Day) happen monthly in the Perl 6 community, with every other one devoted to this repository. It will help the organization of the hackathon if you follow these rules when participating in it.
+
+1. Always work against an issue. If you find some typo or any other kind of error for which an issue does not exist, please create one.
+2. Always work on assigned issues. Assignments get echoed in the [`perl6` IRC channel](https://webchat.freenode.net/?channels=#perl6), so everyone knows someone is working on that.
+3. You can only assign issues to contributors, so if someone who is not a contributor asks to be assigned something to a contributor, please assign it to yourself and do an `@` mention in the issue to the person that is actually doing the work.
+4. Please leave the [good first issues](https://github.com/perl6/doc/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22) until the end of the hackathon; if you find something that can be solved easily in the first hours, please tag it that way and leave it until the end.
+