doc: Mention repo split in the READMEs

Author: MarcoFalke

CONTRIBUTING.md

@@ -84,6 +84,26 @@ To contribute a patch, the workflow is as follows:
   1. Create topic branch
   1. Commit patches
 
+For GUI-related issues or pull requests, the https://github.com/bitcoin-core/gui repository should be used.
+For all other issues and pull requests, the https://github.com/bitcoin/bitcoin node repository should be used.
+
+The master branch for all monotree repositories is identical.
+
+As a rule of thumb, everything that only modifies `src/qt` is a GUI-only pull
+request. However:
+
+* For global refactoring or other transversal changes the node repository
+  should be used.
+* For GUI-related build system changes, the node repository should be used
+  because the change needs review by the build systems reviewers.
+* Changes in `src/interfaces` need to go to the node repository because they
+  might affect other components like the wallet.
+
+For large GUI changes that include build system and interface changes, it is
+recommended to first open a pull request against the GUI repository. When there
+is agreement to proceed with the changes, a pull request with the build system
+and interfaces changes can be submitted to the node repository.
+
 The project coding conventions in the [developer notes](doc/developer-notes.md)
 must be followed.
 

README.md

@@ -25,9 +25,14 @@ information or see https://opensource.org/licenses/MIT.
 Development Process
 -------------------
 
-The `master` branch is regularly built (see doc/build-*.md for instructions) and tested, but is not guaranteed to be
+The `master` branch is regularly built (see `doc/build-*.md` for instructions) and tested, but it is not guaranteed to be
 completely stable. [Tags](https://github.com/bitcoin/bitcoin/tags) are created
-regularly to indicate new official, stable release versions of Bitcoin Core.
+regularly from release branches to indicate new official, stable release versions of Bitcoin Core.
+
+The https://github.com/bitcoin-core/gui repository is used exclusively for the
+development of the GUI. Its master branch is identical in all monotree
+repositories. Release branches and tags do not exist, so please do not fork
+that repository unless it is for development reasons.
 
 The contribution workflow is described in [CONTRIBUTING.md](CONTRIBUTING.md)
 and useful hints for developers can be found in [doc/developer-notes.md](doc/developer-notes.md).