Update windows.md with instructions to bootstrap Bazel.

Fixes #276.

RELNOTES: Experimental Windows support is available.

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MOS_MIGRATED_REVID=115052789

site/docs/windows.md

@@ -6,6 +6,29 @@ title: Windows
 Building Bazel on Windows
 =========================
 
-Building Bazel on Windows is not currently supported, although we are actively
-working on enabling it. If you are interested in current status or ready to
-lend a hand, reach out via bazel-discuss@googlegroups.com.
+Windows support is highly experimental. Known issues are [marked with
+label "Windows"](https://github.com/bazelbuild/bazel/issues?q=is%3Aissue+is%3Aopen+label%3AWindows)
+on github issues.
+
+We currently support only 64bit Windows 7 or higher.
+To bootstrap Bazel on Windows, you will need:
+
+*    Java JDK 8 or later
+*    [msys2](https://msys2.github.io/) (need to be installed at ``C:\msys64\``).
+*    Several MSYS packages (gcc, git, curl): use ``pacman`` command
+     to install those.
+
+To build Bazel:
+
+*    Open MSYS2 shell.
+*    Clone bazel git repository as normal.
+*    Set the environment variables:
+
+```
+export JAVA_HOME="$(ls -d C:/Program\ Files/Java/jdk* | sort | tail -n 1)"
+export TMPDIR=c:/temp
+export BAZEL_SH=c:/msys64/usr/bin/bash.exe
+```
+
+*     Run ``compile.sh`` in Bazel directory.
+*     If all works fine, bazel will be build at ``output\bazel.exe``.