copilot: Include missing dependencies in installation instructions. Refs #591.

The Linux section of the README lists GHC and Cabal as dependencies, but
more packages are needed. Specifically, in a fresh new Ubuntu as
provided via the Docker Hub, users also need to install libz-dev and
pkg-config. The programs alex and happy also need to be available.

This commit adjust the installation instructions for Linux distributions
where Copilot is not directly available as a package to include other
dependencies needed, namely libz-dev, pkg-config, alex and happy.

The text prior to the instructions is also updated to reflect that this
is no longer just the Haskell toolchain but, rather, all dependencies of
Copilot.

Author: ivanperez-keera

copilot/README.md

@@ -102,10 +102,10 @@ Copilot you must install a Haskell compiler (GHC) and the package manager
 Cabal. We currently support all versions of GHC from 8.6.5 to modern versions
 (9.8 as of this writing). You can install the toolchain using
 [ghcup](https://www.haskell.org/ghcup/) or, if you are on Debian/Ubuntu,
-directly with `apt-get`:
+you can use `apt-get` to install all dependencies as follows:
 
 ```sh
-$ sudo apt-get install ghc cabal-install
+$ sudo apt-get install ghc cabal-install alex happy pkg-config libz-dev
 ```
 
 Once the compiler is installed, install Copilot from