RFE: Simple install instructions

[Rob Nagler]

I was trying to install using install.sh, and it did nothing (see radiasoft/zgoubi#90). @rouson suggested I build off master, which did work, but only after I figured that you can do things much more simply.

There are many different install instructions. INSTALL.md starts off with Developer Build and Install with these instructions:

mkdir opencoarrays-build
cd opencoarrays-build
export FC=/path/to/gfortran
export CC=/path/to/gcc
cmake /path/to/OpenCoarrays/source \
  -DCMAKE_INSTALL_PREFIX=/path/to/desired/installation/location
make
make test # optional; verify build works
make install

However, this is atypical and doesn't work with copy and past, especially for the "impatient" like me. I found that this works:

git clone https://github.com/sourceryinstitute/OpenCoarrays
cd OpenCoarrays
mkdir build
cd build
cmake ..
make
make test # optional; verify build works
sudo make install

You could add more advanced instructions after this, but on a typical distro, cmake will find everything automatically. If the user doesn't have sudo privs, you could have another section explaining how to change the install prefix and so on.

[Damian Rouson]

Thanks, @robnagler. I'll make the suggested edits.