Fix install instructions

[apoco]

The provided install instructions do not work.

$ cabal install parsec

Resolving dependencies...
Error: [Cabal-7127]
Cannot build the executables in the package parsec because it does not contain any executables. Check the .cabal file for the package and make sure that it properly declares the components that you expect.

It looks like at some point cabal made it so you've have to use a --lib option to install a library.

[phadej]

cabal install --lib is a 99.9% of time a wrong thing to do. The whole section should be rewritten, or just removed and replaced with

## Adding parsec as a dependency

Add 

```
  build-depends: parsec
```

to your package's `.cabal` file.