Update readme

Change-Id: I12d86b82d245f6acee655f0da5be35bfee2631aa
Reviewed-on: https://gn-review.googlesource.com/1861
Reviewed-by: Petr Hosek <phosek@google.com>
Commit-Queue: Scott Graham <scottmg@chromium.org>

README.md

diff --git a/README.md b/README.md
index 498b017..7027121 100644
--- a/README.md
+++ b/README.md
@@ -3,9 +3,40 @@
 GN is a meta-build system that generates build files for
 [Ninja](https://ninja-build.org).
 
-Get started with:
+## Getting started
 
     git clone https://gn.googlesource.com/gn
     cd gn
     python build/gen.py
     ninja -C out
+
+On Windows, it is expected that `cl.exe`, `link.exe`, and `lib.exe` can be found
+in `PATH`, so you'll want to run from a Visual Studio command prompt, or
+similar.
+
+On Linux and Mac, the default compiler is `clang++`, a recent version is
+expected to be found in `PATH`. This can be overridden by setting `CC`, `CXX`,
+and `AR`.
+
+## Sending patches
+
+GN uses [Gerrit](https://www.gerritcodereview.com/) for code review. The short
+version of how to patch is:
+
+    ... edit code ...
+    ninja -C out && out/gn_unittests
+
+Then, to upload a change for review:
+
+    git commit
+    git push origin HEAD:refs/for/master  # This uploads for review.
+
+When revising a change, use:
+
+    git commit --amend
+    git push origin HEAD:refs/for/master
+
+which will add the new changes to the existing code review, rather than creating
+a new one.
+
+TODO: AUTHORS, CLA, etc.