doc: clarify build instructions

* README: A few fixes.
Explain how to install color support.
* README-hacking: Rename as...
* README-hacking.md: this, and convert to Markdown.
Improve typography.
Improve explanations about update-test.

build-aux/update-test

@@ -1,8 +1,10 @@
 #! /usr/bin/env python
 
 # usage:
-# update-test _build/8d/tests/testsuite.dir/*/testsuite.log
-
+#
+#    update-test _build/8d/tests/testsuite.dir/*/testsuite.log
+#
+# from your source tree.
 import argparse
 import os
 import re