Update GN readme.

tools/gn/README.md

 # What is GN?
 
 GN is a meta-build system that generates
 [NinjaBuild](https://chromium.googlesource.com/chromium/src/+/master/docs/ninja_build.md)
 files. It's meant to be faster and simpler than GYP. It outputs only Ninja build
 files.
 
 ## Why bother with GN?
 
 1. We believe GN files are more readable and more maintainable
    than GYP files.
 2. GN is fast:
   * GN is 20x faster than GYP.
   * GN supports automatically re-running itself as needed by Ninja
     as part of the build. This eliminates the need to remember to
     re-run GN when you change a build file.
 3. GN gives us better tools for enforcing dependencies (see
    `gn check` and the `visibility`, `public_deps`, and `data_deps`
    options for some examples).
 4. GN gives us tools for querying the build graph; you can ask
    "what does X depend on" and "who depends on Y", for example.
 
 ## What's the status?
 
-_as of 7 Oct 2015:_
+_as of March 2016:_
 
-GN is now the default system for Linux, though GYP still works. It
-is mostly complete on Android, ChromeOS, and Windows (apart from NaCl
-support on Windows).
-
-Mac and iOS are making progress, though we still need better support
-for bundles before the major targets like Chrome can link and they get
-to parity w/ the other platforms.
+  * Linux and Android: Complete but not quite all bots have been flipped.
+    Developers should be using GN on these platforms for most work.
+  * Windows and ChromeOS: Should be usable for almost all development. Some
+    bots need to be flipped. cros_sdk builds don't work on ChromeOS yet.
+  * Mac and iOS: Work is in progress, generally not usable for normal
+    development.
 
 ## When are you going to be done?
 
-_as of 7 Oct 2015:_
+_as of March 2016:_
 
-We're currently shooting for having Android, ChromeOS, and Windows
-converted over by the end of 2015, with Mac and iOS following in Q1
-of 2016.
+We expect most work to be finishing up between now and the end of Q2.
 
 ## What does "done" mean?
 
 Ideally we're done when all of the GYP files have been deleted from
 Chromium and no one misses them.
 
 We will be "mostly" done when the following are true:
 
   * All of the bots maintained by the Chrome infra team for Chromium and
     downstream of Chromium have been switched to GN. (Upstream projects
     like Skia and V8 can choose to stay on GYP if they like).
   * Any developer configurations we care about that don't have bots also
     work (Generally speaking, we're aiming to not have any of these.
   * Configurations we care about should have bots to ensure they don't
     break).  We have verified that all of the tests pass.  We have
     verified that the command lines match in the above configurations as
     much as possible, and we accept any differences.  We have reviewed
     any binary differences that result in the official builds and
     accepted them.  The GN files are the "source of truth" for the
     build, and normal chromium developers normally do not need to touch
     GYP files to keep things working.  We have replacements for the
     hybrid "msvs-ninja" and "xcode-ninja" configurations that GYP can
     currently build.
 
 The difference between "mostly done" and "done" exists to cover any
 issues we haven't yet identified :)
 
-We do not currently plan to support full native XCode or Visual Studio
-generation from GN. It is theoretically possible to support such things,
-so we would at least look at patches adding the functionality.
-
 ## How can I help?
 
-Check to see if your targets build under GN yet. If they don't,
-volunteer to help convert them!
+We could espeially use infrastructure/bot, Mac, and iOS help.
 
 Or, look at [the list of open bugs](https://code.google.com/p/chromium/issues/list?can=2&q=label:Proj-GN-Migration%20-type:Project&sort=pri&colspec=ID%20Pri%20Summary%20Type%20OS%20Owner%20Status%20Modified%20Blocking) related to the migration and see if there's anything that catches your fancy.
 
 ## I want more info on GN!
 
 Read these links:
 
   * [Quick start](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/quick_start.md)
   * [FAQ](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/faq.md)
   * [GYP conversion cookbook](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/cookbook.md)
   * [Language and operation details](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/language.md)
   * [Reference](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/reference.md) The built-in `gn help` documentation.
   * [Style guide](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/style_guide.md)
   * [Cross compiling and toolchains](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/cross_compiles.md)
   * [Hacking on GN itself](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/hacking.md)
   * [GNStandalone](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/standalone.md) Standalone GN projects
   * [UpdateGNBinaries](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/update_binaries.md) Pushing new binaries
   * [Check](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/check.md) `gn check` command reference