add write permissions policy

Author: eteq

policies/coordinated-write-permissions.md

@@ -0,0 +1,9 @@
+# Write Permissions for Coordinated Package Repositories
+
+This policy describes the rules and expectations for receiving write permissions for Coordinated Packages (including the Astropy Core Package).
+
+Github write privileges are granted when someone is given a relevant named role. In most cases this is package (or sub-package) maintainer, although other roles (for example, the astropy.org web page maintainer) may also come with write permissions for a specific named repository.  Occasional *temporary* write (or higher) permissions may be grant for specific tasks (for example, a maintainer may be given admin rights to a repo to configure CI for the first time or similar), be such permissions must be done temporarily.
+
+Normal write permissions should be given by assigning sometime to the Github Team that corresponds to that role (For Example, "Astropy Core Maintainers" or "Astropy web site maintainers" for a core maintainer or the astropy.org team, respectively), a duty primarily performed by the Coordination Committee.  Temporary permissions should instead use the "collaborator" feature on Github to make it clear that these permissions are temporary in nature.
+
+Additionally, the granter of permissions (usually the Coordination Committee) should send a message to the new recipient of write permissions the responsibilities and expectations that go with this - a template for this email is available [in this repo](../messages/core_write_access.md). That message may contain a prompt for a response, which should be cced/forwarded to [email protected]