Fix-15074: Added OS info about building from source in README

[goelakash]

#15074 It was mentioned that building via make is not a universal solution. I have added a small change to notify user about which OS' this works with. Should I also add a line wrt to Windows as -

"To build on Windows, see instructions on how to compile from source using Visual Studio or install Cygwin"?
I am not sure whats the correct recommendation, or Julia community's policies about these are.
Please review.

[tkelman]

There's a separate readme with full instructions for building on windows, we have a link to it at https://github.com/JuliaLang/julia#windows but it's a ways down the page. Building with Visual Studio is highly experimental and not something we can officially support right now (see #12056).

[goelakash]

@tkelman How about I add "For details see #link" in there?

[tkelman]

Dunno, there are a lot of build dependencies besides just make. We could say See [Platform-Specific Build Notes](#Platform-Specific-Notes) for more details. or something like that.

[goelakash]

I did add that. Seems ok now.

[tkelman]

The part you added To build on Linux, OSX(with **make** utility installed) or other *nix systems, is a bit overly verbose and strangely formatted. We build on Windows using make as well. I'd say add the short link right after the short sentence the way it was.