Introduction
describe what you will write about. Mention the important points, but don't include any detail. Don't include a table of contents.
Description of a makefile
A high level description. Mention rules, variables, conditional constructions,etc.
Standard rules in debian/rules
Describe each of the standard rules in debian/rules. Check the various devscripts and policy to make sure the rules are truly standard.
Hints to make rules files
Mention dynamically generated rule names, debhelper commands, standard variables exported by the build environment, shell escapes, and other useful tricks
Further reading
Add links to the relevant section of debian policy, the official make manual, any guides on makefiles you found especially good, etc.