Add manual build instructions + windows script

[elig0n]

I was trying to figure out how to load the source tree unpacked into Chrome and eventually found that there is some build process. I ended up needing to manually run git clone --depth 1 https://github.com/uBlockOrigin/uAssets.git in the directory above the package and run tools/make-chromium.sh from inside WSL to finally get a loadable dist/build/uMatrix.chromium unpacked distribution that Chrome can load.
This wasn't specified in any README file. Adding windows batch scripts might also be appropriate.

Thanks for the developers of this wonderful extension!