Update README.md to add windows docs

[mondus]

Adds details of

• How to install Intel Fortran in Windows
• Details on obtaining the CMAKE_PREFIX_PATH
• Cross platform build and install commands

[jatkinson1000]

Thanks @mondus a couple of small tweaks and a question about the install command.
I've forwarded it to the user and will see how she gets on - using cmake --build rather than make may be important!

[jatkinson1000]

This is WIP with #50
Once that is resolved with the user we will update here accordingly and then merging will close #50

[jatkinson1000]

I think this is good to go now (correspondent from #50 has gone quiet, but it works for us).
Since I have done a lot of this PR I will let @TomMelt do a final review for approval.

[TomMelt]

looks good to me. Just a couple of questions

[TomMelt]

just added a note about loading intel compilers. If @mondus is happy then I think we can merge it in.

[jatkinson1000]

just added a note about loading intel compilers. If @mondus is happy then I think we can merge it in.

@TomMelt Would it be worth extending that by telling users that this can usually be done by running C:\Program Files (x86)\Intel\oneAPI\setvars from Cmd?

[mondus]

Tested in Windows. All good. Happy to approve. One minor suggestion comment to add a link to Intel guide on setvars.

[jatkinson1000]

Thanks @mondus, that also addresses my comment above but in an even safer/better way than suggested, thanks.
I'll approve and merge now (@mondus has confirmed offline that he is happy with the PR).

[jatkinson1000]

Thanks all who contributed here, I'm now happy to go ahead so will merge in a moment.

Changes have been made as appropriate.