all: accept Github PRs

Change-Id: I804784c97e4133f7ea3e0757e5591915331bb346
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2289
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>

README.md

@@ -130,7 +130,7 @@ To contribute, please read the [Contribution Guide](./doc/contribute.md).
 To report issues or make a feature request, use the
 [issue tracker](https://github.com/cuelang/cue/issues).
 
-##### Note that we do not accept pull requests and that we use the issue tracker for bug reports and proposals only.
+Changes can be contributed using Gerrit or Github pull requests.
 
 Unless otherwise noted, the CUE source files are distributed
 under the Apache 2.0 license found in the LICENSE file.

doc/contribute.md

@@ -126,6 +126,10 @@ completed and update the `AUTHORS` file.
 
 ### Step 2: Configure git authentication
 
+The remaining two steps only apply if you wish to contribute through Gerrit,
+which is the source of truth for the CUE project. You can also send Pull
+Requests to the mirror at https://github.com/cuelang/cue.
+
 The main CUE repository is located at
 [cue.googlesource.com](https://cue.googlesource.com),
 a Git server hosted by Google.
@@ -316,7 +320,8 @@ Remember you can always visit Gerrit to see the fine-grained review.
 
 ## Sending a change via Gerrit
 
-It is not possible to fully sync Gerrit and GitHub, at least at the moment,
+It is not possible to fully sync Gerrit and GitHub,
+although things are improving,
 so we recommend learning Gerrit.
 It's different but powerful and familiarity
 with help you understand the flow.