Move the toolchain build instructions from mingw-builds to mingwpy's README.md

[ogrisel]

I think we should move the instructions from:

https://github.com/mingwpy/mingw-builds/blob/mingwpy-dev/mingwpy_readme.md

into the README.md of the main mingwpy repo.

This way we keep our mingwpy-dev branch of mingw-builds more inline with upstream and centralise the technical instructions to build the mingwpy toolchains in our own repo.

[techtonik]

I thought that README is the main file. Now it looks like entrypoint is different.

[ogrisel]

README.md is used by github to build the home page of a repo. At this point the mingwpy-dev branch of our clone of the mingw-builds has mingwpy specific instructions hidden in a file named mingwpy_readme.md.

We don't want to pollute upstream mingw-builds with minwpy-specific stuff so I think it's better to gather those information in the toplevel README.md file of our own mingwpy repository.

This way the mingwpy-dev branch of our clone of the mingw-builds can be more in line with upstream mingw-builds. This branch can be considered as temporary container of patches that we plan to submit upstream to make the mingw-builds scripts generic enough for our usecase will not imposing any mingwpy specific documentation on them.

At the same time it will make it easier for people interested specifically in building the mingwpy toolchain from source to find the build instructions.

[ogrisel]

This is related to the intention to contribute to upstream mingw-build as tracked in #5 so as to keep the long term maintenance efforts of mingwpy to a minimum.

[techtonik]

I added entrypoint README.md in PR above, but I haven't got time to read and validate the info about branch layout and handling of patch queues. Need more round tuits.

[carlkl]

I moved the build instructions as requested.

[ogrisel]

Thanks!