Add link on how to request to be whitelisted for libphonenumber-supporting-docs

CONTRIBUTING.md

@@ -37,7 +37,8 @@ If the evidence is not publicly available online, make sure that you have the ri
 
 **By signing the CLA, you confirm that you have the rights to share the information with us, and that we may use, modify, reproduce, publicly display, and distribute the information in accordance with the library's [open source license](https://github.com/googlei18n/libphonenumber/blob/master/LICENSE).**
 
-Once you have signed the CLA, you can request to be whitelisted to send emails to the [libphonenumber-supporting-docs](mailto:libphonenumber-supporting-docs@googlegroups.com) mailbox. Put the GitHub issue number in the subject line and attach the supporting documents. Only the core libphonenumber team has access to view emails sent to this mailbox.
+Once you have signed the CLA, you can [request to be whitelisted]
+(https://groups.google.com/forum/#!managemembers/libphonenumber-supporting-docs) to send emails to the [libphonenumber-supporting-docs](mailto:libphonenumber-supporting-docs@googlegroups.com) mailbox. Put the GitHub issue number in the subject line and attach the supporting documents. Only the core libphonenumber team has access to view emails sent to this mailbox.
 
 ## Filing a code issue