Move model section from issue template to README.md (#421)

* Update custom.md

* Removed Model section as it is better placed in README.md

* Updates to README.md model section

* Inserted text that was removed from  issue template about obtaining models from FB and links to papers describing the various models

* Removed IPF down links for the Alpaca 7B models as these look to be in the old data format and probably shouldn't be directly linked to, anyway

* Updated the perplexity section to point at Perplexity scores #406 discussion