Add rules.

rules.md

+# Rules
+
+If you're contributing or you're an member, you should comply these rules:
+
+* You can assign yourself to only an issue.
+* After assigning yourself, in 24 hours you should write an expiration date deadline.
+* Unless you're a member, you should write a comment to the issue; a member will assign you.
+
+### Penalties
+* If you don't give an expiration date deadline in 24 hours, you will get unassigned from issue.