Main front-page Readme should be updated with up-to-date installation instructions

[sit23]

Description

The current Readme.md file in the top directory gives information on compiling for the first time using Intel compilers, but it would be sensible to change this to reflect the gfortran compilation that's possible with the ubuntu_conda env. I'm happy to do this, but don't have the time just now. Leaving this here to remind me to do it.

[daw538]

Before this is completely addressed, is it worth checking if the fix applied in #236 needs applying to the other environment files? I think it only has been done to ubuntu_conda so far.

[sit23]

@daw538 - yes, we should make that fix to the gfortran environment file, and others should be checked too.