Improve documentation for sandbox config

[mietek]

Currently, for cabal build and related commands (such as cabal test, cabal repl, and cabal run) to use the Halcyon sandbox, cabal-install must be pointed to it in one of the following ways:

1. Specify a command-line option:

   $ cabal --sandbox-config-file="${HALCYON_BASE}/sandbox/cabal.sandbox.config" build
   

2. Set an environment variable:

   $ export CABAL_SANDBOX_CONFIG="${HALCYON_BASE}/sandbox/cabal.sandbox.config"
   $ cabal build
   

3. Create a symlink:

   $ ln -fs "${HALCYON_BASE}/sandbox/cabal.sandbox.config" .
   $ cabal build
   

Soon, the halcyon activate command will set CABAL_SANDBOX_CONFIG automatically (mietek/halcyon#46). This should be made clear in the documentation.