Update docs for Bash on Ubuntu on Windows (#5671)

Add docs for compiling Swift on Windows Subsystem for Linux

Author: hughbe

Diff for: docs/Windows.md

@@ -0,0 +1,57 @@
+# Getting Started with Swift on Windows
+
+## MSVC
+To be filled in.
+
+## Windows Subsystem for Linux (WSL)
+- Note that all compiled Swift binaries are only executable within Bash on Windows and are Ubunutu, not Windows, executables.
+- Make sure to run all commands from Bash, or the project won't compile.
+
+###  1. Install WSL
+Install and run the latest version of [Bash on Ubuntu on Windows](https://msdn.microsoft.com/en-gb/commandline/wsl/about) installed on your PC.
+```
+bash
+```
+
+### 2. Install dependencies
+Install the developer dependencies needed to compile the Swift project. These are identical to the Ubuntu dependencies, with the addition of `make`.
+```bash
+sudo apt-get install git make cmake ninja-build clang python uuid-dev libicu-dev icu-devtools libbsd-dev libedit-dev libxml2-dev libsqlite3-dev swig libpython-dev libncurses5-dev pkg-config libblocksruntime-dev libcurl4-openssl-dev
+```
+
+### 3. Upgrade clang
+Install a version of clang with C++ 14 support - the default version of clang on WSL results in linker errors during compilation.
+```bash
+sudo apt-get install clang-3.6
+sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-3.6 100
+sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-3.6 100
+```
+
+### 4. Upgrade CMake
+Install the latest version of CMake - Swift uses new CMake features such as `IN_LIST` and won't build without these features.
+```bash
+wget http://www.cmake.org/files/v3.5/cmake-3.6.2.tar.gz
+tar xf cmake-3.6.2.tar.gz
+cd cmake-3.6.2
+./configure
+make
+sudo make install
+sudo update-alternatives --install /usr/bin/cmake cmake /usr/local/bin/cmake 1 --force
+cmake --version # This should print 3.6.2
+```
+
+### 6. Clone and build the Swift project
+```bash
+mkdir swift-source
+cd swift-source
+git clone https://github.com/apple/swift.git
+./swift/utils/update-checkout --clone
+./swift/utils/build-script -r
+```
+### 7. Hello, Windows (Subsystem for Linux)
+```bash
+cd ./build/Ninja-RelWithDebInfoAssert/swift-linux-x86_64/bin # This path may depend on your build configuration
+echo 'print("Hello, Windows")' >> test.swift
+swiftc test.swift
+./test # Hello, Windows
+```