Add instructions for getting AllenNLP to run on Windows

It's currently a pain, apparently, and I'm not sure how much work we should do to support it (if you're reading this and want Windows support, add a comment please). But @OyvindTafjord was able to get it actually working by modifying some things:

1. Got pytorch from 'conda install -c peterjc123 pytorch cuda80' (https://github.com/pytorch/pytorch/issues/494)
2. Pip install of spacy (and other things) required some VS2015 library stuff (had to be same version as used to compile python).
3. Pip install of sanic failed (because a dependency, uvloop, is not available for Windows) [ got around it by remove serve from commands ]
4. In general 'pip install -r requirements.txt' worked weirdly and I ended up running each line individually, I didn't dig into why that was.
5. Downloading dataset from S3 failed, I think because of non-allowed double quotes in the filename (from the etag) [ got around it by removing etag part of filename]
6. Download also threw exception because it couldn't remove the temporary file, seems like Windows hangs onto the file for a bit, a 1 second pause didn't help [ got around by catching that exception, deleting the files manually later ]
7. Had issues getting appropriate tensorflow/tensorboard installed where SummaryWriter was found [ got around it by instead using TensorboardX which somebody made for this purpose ]

I'm going to mark this issue as very low priority, until we hear people actually asking for us to support this. For now, the instructions will just live in this issue.