Add README for documentation.

[drroe]

Adds a README describing the files in the documentation subdirectory doc.

@rodrigogalindo this is also being used as an independent test of #1150 to see if the Appveyor/Jenkins test failures can be replicated.

[drroe]

The failing appveyor mingw tests are all related to FFT. I wonder if the recent update from mingw 14 to mingw 15 is the cause. Either mingw 15 is broken for the fortran-based PubFFT, it's not compiling it correctly because some flag(s) isn't set, or there is some long-hidden bug in PubFFT that is being exposed. Might be worth it to try to get pacman to download fftw.

[drroe]

The issue here is pacman:

gpg: refreshing 8 keys from hkp://pool.sks-keyservers.net
gpg: keyserver refresh failed: Try again later
==> ERROR: A specified local key could not be updated from a keyserver.
warning: mingw-w64-x86_64-gcc-9.1.0-3 is up to date -- skipping
warning: mingw-w64-x86_64-gcc-fortran-9.1.0-3 is up to date -- skipping
warning: mingw-w64-x86_64-ncurses-6.1.20190630-1 is up to date -- skipping
warning: mingw-w64-x86_64-readline-8.0.000-4 is up to date -- skipping
warning: diffutils-3.7-1 is up to date -- skipping
resolving dependencies...
looking for conflicting packages...
Packages (17) mingw-w64-x86_64-brotli-1.0.7-1  mingw-w64-x86_64-c-ares-1.15.0-1  mingw-w64-x86_64-curl-7.65.3-1  mingw-w64-x86_64-hdf5-1.10.5-1  mingw-w64-x86_64-jansson-2.12-1  mingw-w64-x86_64-jemalloc-5.2.0-1  mingw-w64-x86_64-libidn2-2.2.0-1  mingw-w64-x86_64-libmetalink-0.1.3-3  mingw-w64-x86_64-libpsl-0.21.0-1  mingw-w64-x86_64-libssh2-1.9.0-1  mingw-w64-x86_64-libunistring-0.9.10-1  mingw-w64-x86_64-nghttp2-1.39.1-1  mingw-w64-x86_64-szip-2.1.1-2  mingw-w64-x86_64-arpack-3.7.0-2  mingw-w64-x86_64-fftw-3.3.8-1  mingw-w64-x86_64-netcdf-4.7.0-1  mingw-w64-x86_64-openblas-0.3.6-2
Total Download Size:    25.00 MiB
Total Installed Size:  192.63 MiB
:: Proceed with installation? [Y/n] warning: no /var/cache/pacman/pkg/ cache exists, creating...
:: Retrieving packages...
error: failed retrieving file 'mingw-w64-x86_64-c-ares-1.15.0-1-any.pkg.tar.xz' from repo.msys2.org : The requested URL returned error: 404

Probably mad I've done like 10 builds in 24 hours. I'll wait a bit and try one more time. Then I'm merging.

[drroe]

Jenkins failure is the symmetric RMSD Heisenbug.

[drroe]

run jenkins

[drroe]

It might be time to really get rid of appveyor.