Adds help command

[hasukmistry]

Adds help command in make file.

[Robinlovelace]

This is cool!

Just tried it:

git checkout -b hasukmistry-feature/add-make-help-target main
git pull git://github.com/hasukmistry/geocompr.git feature/add-make-help-target
make help

And I get:

make help

Usage:
  make <target>

Targets:
  build                 Make Build
  pdf                   Render book in pdf
  md                    Generate Markdown
  install               Perform install
  deploy                Perform deployment
  help                  List available commands and their usage