Makefile: to generate the html files

Makefile

-1_understanding_a_computer.html: 1_understanding_a_computer.md pandoc.css
-	pandoc -s --toc -c pandoc.css 1_understanding_a_computer.md -o 1_understanding_a_computer.html
+all: index.html 1_understanding_a_computer.html 2_first_steps_in_a_terminal.html
+
+index.html: index.md github-pandoc.css
+	pandoc -s -c github-pandoc.css index.md -o index.html
+
+1_understanding_a_computer.html: 1_understanding_a_computer.md github-pandoc.css
+	pandoc -s --toc -c github-pandoc.css 1_understanding_a_computer.md -o 1_understanding_a_computer.html
+
+2_first_steps_in_a_terminal.html: 2_first_steps_in_a_terminal.md github-pandoc.css
+	pandoc -s --toc -c github-pandoc.css 2_first_steps_in_a_terminal.md -o 2_first_steps_in_a_terminal.html