Update installation instructions

[joaomcteixeira]

Updates installation instructions after #21:

• updates commands to new main branch name
• more general cns executable name
• adds conda yml
• facilitates source env

[rvhonorato]

Still think we should remove lightdock from the dependencies; will all the supported modules be part of dependencies? what if next we add PatchDock, maybe ZDOCK, or even GROMACS, do they also needs to be installed so that you can run haddock? I don't think it makes much sense, but up to you.

[amjjbonvin]

Probably makes more sense to have it as an option

[joaomcteixeira]

I understand your point. Well, at the current state those questions are difficult to answer in my opinion. But I understand your point. I will set lightdock as an optional dependency in another pull request and create the infrastructure to accommodate other optional dependencies as well. But I won't change them now in this PR not to mix too many things. 👍