[DOC DAY] Updating installation instructions

[spencerp]

Patch description

• Add some missing requirements
• Fix requirements section link in README

Testing steps

parlai display_data -t squad 
-> Ran fine

python parlai/scripts/safe_interactive.py -t blended_skill_talk -mf zoo:blender/blender_90M/model
-> Ran fine

$ python -m parlai.scripts.train_model -t personachat -m transformer/ranker -mf /tmp/model_tr6 --n-layers 1 --embedding-size 300 --ffn-size 600 --n-heads 4 --num-epochs 2 -veps 0.25 -bs 64 -lr 0.001 --dropout 0.1 --embedding-type fasttext_cc --candidates batch  
->  ModuleNotFoundError: No module named 'torchtext'

Added --tensorboard-log true 
->  ImportError: Please run `pip install tensorboard tensorboardX`.

$ parlai display_data -t dialogue_safety
->  ImportError: Please install pandas by running `pip install pandas`

pip freeze gave me these versions:

pandas==1.1.1
tensorboard==2.3.0
tensorboardX==2.1
torchtext==0.7.0

I then created a new environment, ran python setup.py develop, and verified that all of the above commands now work "out fo the box".