Docs: Add OSX instructions for building You-Complete-Me.

Without the --system-clang flag, ycmd fails to find the system headers,
which severely limits its usefulness. The flag is documented as
unsupported, but it's recommended by the maintainers on OSX:
https://github.com/Valloric/YouCompleteMe/issues/2536#issuecomment-333909807

Change-Id: I93aeb12656d32e0fa2099aa0c703fcb6f7154bfb
Reviewed-on: https://chromium-review.googlesource.com/925102
Reviewed-by: Rouslan Solomakhin <[email protected]>
Reviewed-by: Jianpeng Chao <[email protected]>
Commit-Queue: Victor Costan <[email protected]>
Cr-Commit-Position: refs/heads/master@{#537864}
diff --git a/docs/atom.md b/docs/atom.md
index 1499169d..9638a4e 100644
--- a/docs/atom.md
+++ b/docs/atom.md
@@ -46,6 +46,12 @@
 $ ./build.py --clang-completer
 ```
 
+On Mac, replace the last command above with the following.
+
+```
+$ ./build.py --clang-completer --system-libclang
+```
+
 ## JavaScript lint
 
 Install JavaScript linter for Blink layout tests.