Copy some guidance from teh sites page to src/README about when to add new directories.

Change-Id: Ibd9edc2f826f3a6a0cd4014fccf9308fd3791285
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2080076
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745596}

README.md

@@ -9,3 +9,8 @@ Documentation in the source is rooted in [docs/README.md](docs/README.md).
 
 Learn how to [Get Around the Chromium Source Code Directory Structure
 ](https://www.chromium.org/developers/how-tos/getting-around-the-chrome-source-code).
+
+For historical reasons, there are some small top level directories. Now the
+guidance is that new top level directories are for product (e.g. Chrome,
+Android WebView, Ash). Even if these products have multiple executables, the
+code should be in subdirectories of the product.