Add installation instructions through conda-forge

[h-vetinari]

After almost a year of operating the various feedstocks for the allennlp-ecosystem, I think it's justified to add the installation instructions for conda-forge also in the main repo.

I've allowed myself to place the conda install first, because (if only measured by number of lines and/or instructions), it is much easier than the installation through pip.

However, I'm happy to adapt this in any way necessary so that people here are happy with it.

Git messes up the diff quite a bit, so I've broken down the commits into more digestible pieces.

[dirkgr]

Big fan! Thank you!

[dirkgr]

Does this work already with https://github.com/allenai/tango? Can we do it there too?

[h-vetinari]

Does this work already with https://github.com/allenai/tango? Can we do it there too?

Tango is already available. :)

Happy to do it there too, but wanted to get some overall feedback first.

[h-vetinari]

One more thought: generally, we'll be fast to release for conda-forge, but I opened a tracker for new dependencies on the feedstock to cut down on the lag where possible: conda-forge/allennlp-feedstock#32