confirm Windows build instructions and update README.md

[ericwa]

I am assuming it's possible to build DV on Windows by installing

• gcc
• make
• patch
• bash
• git

from Cygwin and then running the init.sh script, then proceeding with ndk-build.

Need to confirm if this actually works, or is there an easier way?

[sss]

I found https://wiki.videolan.org/Win32CompileMSYS as a useful starting point.

I tested the following recipe on Windows 10.

1. Install MSYS2 from http://www.msys2.org/
2. Run pacman -Syu on MSYS2 terminal to update the package database and core system packages
3. Run pacman -Su on MSYS2 terminal to update the rest
4. Run pacman -S make patch bash git gcc to fulfill our dependencies
5. Run ./init.sh in the root of our repository
6. Run ndk-build.cmd in document-viewer directory of of our repository