Improve onboarding experience

Edited the README to have a linear path to installation.
Also made scripts to run the executables without having to worry
whether they are on our path.

bin/cli

+# The following line is more portable than just /bin/bash:
+#!/usr/bin/env bash
+
+# A couple hygienic options
+set -e -u
+
+# The following command will run `cabal run gargantext-cli --` followed by the
+# options provided by the user, from inside a Nix shell. For instance,
+# if the user types
+# $ ./bin/cli someCommand "some string argument"
+# the following will be run from inside a Nix shell:
+# $ cabal run gargantext-cli -- someCommand "some string argument"
+# It's a little convoluted because we want to keep spaces that were enclosed in
+# quotes or escaped by the user.
+nix-shell --run "$(printf "%q " cabal run gargantext-cli -- "$@")"

bin/run

+# The following line is more portable than just /bin/bash:
+#!/usr/bin/env bash
+
+# A couple hygienic options
+set -e -u
+
+echo "Launching Gargantext..."
+nix-shell --run "cabal run gargantext-server -- --run Prod --toml gargantext-settings.toml"