Mention hpack in cabal-install instructions

[MichaelBergquistSuarez]

Since 19e75f0, I think.

cabal install results in:
cabal: Error reading local package.

I had to revert back to tag 0.12.3 to be able to install

[ethercrow]

My guess would be that yi.cabal file is missing until you run hpack first.

[MichaelBergquistSuarez]

Alright, thanks. I think the install instructions should be updated with how to do that. Currently, hpack is only mentioned breifly and the commands for installing are:

$ git clone https://github.com/yi-editor/yi
$ cd yi
$ cabal update
$ cabal install

I haven't used hpack but I suppose there should be a hpack command after cd yi(?)