Modify the structure of README.md

Instead of having exercises in their own separate section,
they are now part of the main topics sections.

This should make the learning process easier to follow
where users have both exercises and questions in the same
place.

In addition, added a couple of new questions & exercises
and modified existing scripts accordingly.

exercises/git/solutions/commit_01_solution.md

@@ -0,0 +1,10 @@
+## Git Commit 01 - Solution
+
+```
+mkdir my_repo && cd my_repo
+git init
+echo "hello_commit" > file
+git add file
+git commit -a -m "It's my first commit. Exciting!"
+git log
+```