Document how to make a codelist for high cost drugs

[brianmackenna]

There is currently no documentation about how to make high cost drugs codelists.

As it is not like the other coding systems it would be good to document on OpenCodelists (or main OS docs site if more appropriate)

In this discussion forum @rose-higgins has shared a gdoc with instructions. @rc16 has tested and thinks they basically work (@rc16 will add some comments to the gdoc about minor things).

I think adding these instructions for now are a good MVP.