Development instructions on website don't match Readme

The build instructions are quite similar however they are different. Also I have never used Haskell before but I want implement a custom implicit function, i.e. spherical harmonics.

Also it appears that neither work as is on ubuntu 18.04

e.g.

using the website instructions the steps fail due to issues with the plugin dependency
```
cabal install plugins
Resolving dependencies...
Configuring haskell-src-1.0.3.0...
Failed to install haskell-src-1.0.3.0
Build log ( /home/krs/.cabal/logs/haskell-src-1.0.3.0.log ):
cabal: Entering directory '/tmp/cabal-tmp-3835/haskell-src-1.0.3.0'
Configuring haskell-src-1.0.3.0...
cabal: The program 'happy' version ==1.19.* is required but it could not be
found.
cabal: Leaving directory '/tmp/cabal-tmp-3835/haskell-src-1.0.3.0'
cabal: Error: some packages failed to install:
haskell-src-1.0.3.0 failed during the configure step. The exception was:
ExitFailure 1
plugins-1.6.0 depends on haskell-src-1.0.3.0 which failed to install.

```

using the README instructions the process fails at install

```
cabal install
Resolving dependencies...
Configuring implicit-0.3.0.0...
Building implicit-0.3.0.0...
Failed to install implicit-0.3.0.0
Build log ( /home/krs/.cabal/logs/implicit-0.3.0.0.log ):
cabal: Entering directory '.'
Configuring implicit-0.3.0.0...
Building implicit-0.3.0.0...
Preprocessing library implicit-0.3.0.0...

: error:
Failing due to -Werror.

on the commandline: warning:
unrecognised warning flag: -Wmissing-export-lists

on the commandline: warning:
unrecognised warning flag: -Wmissing-home-modules
cabal: Leaving directory '.'
cabal: Error: some packages failed to install:
implicit-0.3.0.0 failed during the building phase. The exception was:
ExitFailure 1

```