migrate GN docs from the wiki to the repo.

tools/gn/docs/standalone.md

+# Introduction
+
+This page is about how to design a project that can build independently
+with GN but also be brought into the Chrome build.
+
+GN is in principle no different than GYP in that there is some core
+configuration that must be the same between both the standalone build
+and the Chrome build. However, GN is much more explicit in its naming
+and configuration, so the similarities between the two builds are also
+much more explicit and there is less flexibility in how things are
+configured.
+
+# What you need for a minimal GN build
+
+Requirements:
+
+  * A master build config file. Chrome's is `//build/config/BUILDCONFIG.gn`
+  * A separate build file for the toolchain definition. It's not a good idea
+    to put these in a BUILD.gn file shared with any target definitions for
+    complex reasons. Chrome's are in `//build/toolchain/<platform>/BUILD.gn`.
+  * A `BUILD.gn` file in the root directory. This will be loaded after the
+    build config file to start the build.
+
+You may want a `.gn` file in the root directory. When you run GN it
+recursively looks up the directory tree until it finds this file, and it
+treats the containing directory as the "source root". This file also
+defines the location of the master build config file:
+
+  * See Chrome's `src/.gn` file.
+  * Unlike Chrome, you probably don't need to define a secondary root.
+  * see `gn help dotfile` for more.
+
+Adding a `.gn` file in a repository that is pulled into Chrome means
+that then running GN in your subdirectory will configure a build for
+your subproject rather than for all of Chrome. This could be an
+advantage or a disadvantage.
+
+If you would rather avoid using this file, you can use the command-line
+flags `--root` and `--dotfile` to set these values.
+
+# How the standalone and Chrome builds interact

[Nico, 2017/03/20 14:19:24]

This looks truncated. Pasteboard mishap?

(I noticed 'cause some internal doc --
https://docs.google.com/document/d/1i9MsSl6Y14dG0kA8FFcNBaFdMoDee2kUPg3pEH7HK...
-- pointed out that this file here isn't super helpful.)

It probably should mention tools/gn/example at least, but if you know how to
restore what was originally in this document on the wiki that'd probably be
best.