docs: improve windows.md

This add a lot of steps that were missing and makes it possible to build on Windows.

License: MIT
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>

docs/windows.md

 # Building on Windows
 
+## Install Git For Windows
+
+As Git is used by the Go language to download dependencies, you need
+to install Git, for example from http://git-scm.com/.
+
+You also must make sure that the directory that contains the Git For
+Windows binary is in the Path environment variable. Note that Git For
+Windows has a 'git' binary in a 'Git\bin' directory and another one in
+a 'Git\cmd' directory. You should only put the 'Git\cmd' directory in
+the Path environment variable.
+
+## Install Go
+
+Please install the Go language as explained on
+https://golang.org/doc/install.
+
+You must make sure that the GOROOT environment variable is set and
+that the %GOROOT%/bin directory is in the Path environment variable.
+
+The GOPATH environment variable should also be set to a directory that
+you have created, and the %GOPATH/bin directory should also be in the
+Path environment variable.
+
+## Download go-ipfs and its dependencies
+
+The following commands should download or update go-ipfs and its
+dependencies:
+
+```sh
+go get -u github.com/whyrusleeping/gx
+go get -u github.com/whyrusleeping/gx-go
+go get -u github.com/ipfs/go-ipfs
+cd %GOPATH%/src/github.com/ipfs/go-ipfs
+gx --verbose install --global
+```
+
+If you get authentication problems with Git, you might want to take a
+look at
+https://help.github.com/articles/caching-your-github-password-in-git/
+and use the suggested solution:
+
+```sh
+git config --global credential.helper wincred
+```
+
 ## Build
 
 Fuse is not supported for Windows, so you need to build IPFS without Fuse:
@@ -10,4 +55,4 @@ go build -tags nofuse ./cmd/ipfs
 
 ## TODO
 
-Add more instructions on setting up the golang environment on Windows
+Fix the "Build" section to pass the current commit as the Makefile does.