Update documentation on labels (#113)

Updated for external/internal issues: updated `ready for Dev` --> `external contributors welcome` and `forem team` --> `internal team only`

docs/contributing-guide/forem.md

@@ -20,11 +20,11 @@ contact the team with our [abuse report form](https://dev.to/report-abuse).
 ### Where to contribute
 
 All [issues](https://github.com/forem/forem/issues) labeled
-[ready for dev](https://github.com/forem/forem/issues?q=is%3Aissue+is%3Aopen+label%3A%22ready+for+dev%22)
+[external contributors welcome](https://github.com/forem/forem/labels/external%20contributors%20welcome)
 and
 [bug](https://github.com/forem/forem/issues?q=is%3Aissue+is%3Aopen+label%3Abug)
 are up for grabs. Please note that issues with the
-[Forem team](https://github.com/forem/forem/labels/Forem%20team) label are
+[internal team only](https://github.com/forem/forem/labels/Internal%20team%20only) label are
 internal tasks that will be completed by a Forem
 [core team member](https://github.com/forem/forem/#core-team).
 
@@ -188,4 +188,4 @@ don't force push or worry about squashing your commits.
 Force pushing (despite being useful) has some drawbacks. GitHub doesn't always
 keep the review history, which results in lost context for the reviewers.
 
-We squash every PR before merging, so there is no need to force push!
+We squash every PR before merging, so there is no need to force push!