README needs an update with guidance for usage of wheels

[rgommers]

I was pretty sure that we had guidance somewhere to not depend on the wheels on PyPI directly, but it's not in this repo (or I'm really bad at searching today). We should add the following prominently to the README:

1. The primary purposes of the scipy-openblas32 and scipy-openblas64 wheels are:
   • (a) to use them as build and runtime dependencies in CI and local development for NumPy and SciPy
   • (b) to be vendored into NumPy and SciPy wheels
   • (c) possibly, in the future, being used as runtime dependencies for NumPy and/or SciPy
2. Other Python projects are also welcome to use these wheels for 1(a) and 1(b).
   • Please note that there is no strong guarantee of backwards compatibility for the symbol names nor the small Python API shipped in the wheels to enable linking against the shared library. If you want to use them, you should probably use == pins in the relevant CI/lock files, like NumPy and SciPy also do.
3. <warning-directive> please do not add a runtime dependency on these wheels if you're not NumPy or SciPy. This is not supported and likely to lead to breakage or symbol conflicts due to either changes in this repository or due to NumPy or SciPy starting to depend on a particular version of this package.

[mattip]

Take a look now?

[rgommers]

Looks good, thanks for taking care of that!