Auto merge of #43886 - oli-obk:clippy, r=nrc

Add clippy as a submodule

~~This builds clippy as part of `./x.py build` (locally and in CI).~~

This allows building clippy with `./x.py build src/tools/clippy`

~~Needs https://github.com/nrc/dev-tools-team/issues/18#issuecomment-322456461 to be resolved before it can be merged.~~ Contributers can simply open a PR to clippy and point the submodule at the `pull/$pr_number/head` branch.

This does **not** build clippy or test the clippy test suite at all as per https://github.com/nrc/dev-tools-team/issues/18#issuecomment-321411418

r? @nrc

cc @Manishearth @llogiq @mcarton @alexcrichton

1 files changed, 26 insertions, 0 deletions

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index c424ca7ab0..cdac8e696c 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -298,6 +298,32 @@ Speaking of tests, Rust has a comprehensive test suite. More information about
 it can be found
 [here](https://github.com/rust-lang/rust-wiki-backup/blob/master/Note-testsuite.md).
 
+### External Dependencies
+
+Currently building Rust will also build the following external projects:
+
+* [clippy](https://github.com/rust-lang-nursery/rust-clippy)
+
+If your changes break one of these projects, you need to fix them by opening
+a pull request against the broken project. When you have opened a pull request,
+you can point the submodule at your pull request by calling
+
+```
+git fetch origin pull/$id_of_your_pr/head:my_pr
+git checkout my_pr
+```
+
+within the submodule's directory. Don't forget to also add your changes with
+
+```
+git add path/to/submodule
+```
+
+outside the submodule.
+
+It can also be more convenient during development to set `submodules = false`
+in the `config.toml` to prevent `x.py` from resetting to the original branch.
+
 ## Writing Documentation
 
 Documentation improvements are very welcome. The source of `doc.rust-lang.org`