[next2] update README.md

[HeikoAdams]

Please update README.md with dependencies and commands for building next2-branch successfully.

[Vistaus]

You need to build gtk and vala from the latest git master. After that, it's basically:

mkdir build && cd build
meson .. --prefix=/usr (or leave out the prefix if you want)
ninja -j3 && sudo ninja install

(use one & if on fish instead of bash)

But while adding that to the readme is trivial, truth is that Corebird will only work for two more months anyway:
https://www.patreon.com/posts/corebirds-future-18921328

But of course, I'll leave the decision to baedert ;)

[baedert]

The tricky part is just that you need to build gtk+ and vala from their respective master branches, the other dependencies should be the same and build instructions with meson/ninja are already in the readme.