Update build instructions

[charlesangus]

Build instructions on main page are out of date. These may be helpful in updating them:

https://vfxplatform.com/#footnote-gcc6

https://groups.google.com/g/vfx-platform-discuss/c/_-_CPw1fD3c

[jonassorgenfrei]

Will do this !

[jonassorgenfrei]

Adjusted the readme here:
https://github.com/jonassorgenfrei/DeepC

Will merge this in on the next bigger PR.