Update windows.md with instructions to bootstrap Bazel.
Update windows.md with instructions to bootstrap Bazel.

Fixes #276.

RELNOTES: Experimental Windows support is available.

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MOS_MIGRATED_REVID=115052789

bazelbuild/bazel-website,davidzchen/bazel-website,bazelbuild/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website