Further separate the compilation and linking in recitation 1. Add README.

I rewrote recitation 1 to separate compilation and linking. i.e., you compile main.c before you even write `add()`) to emphasize the point that compiling only cares about the prototype, not the definition. 

Added a README with a short summary and links to each recitation. 

Some miscellaneous cleanup to recitations 3, 5, and 7. 

Renamed sample-makefile to sample-Makefile to help emphasize the conventional naming of Makefile.