For an automatically generated index of what you can find in sources/contrib see: Contrib index.
To get your own directory in sources/contrib to upload your own code see how to contribute.
An INDEX file in a contrib directory can contain entries of the following format:
package_name: Description here.
Where 'package_name' is the full name of a file in that directory. This is used to build the contrib index.
Another option is to use Federico G. Benavento's contrib package management system. To install this system, issue
/n/sources/contrib/fgb/root/rc/bin/contrib/install fgb/contrib
The Contrib index tells you the names of all users that have packages available under this system. You can list all the packages of a given user by stating
contrib/list user
or details of a specific package with
contrib/list user/package
To install a package, issue
contrib/install user/package
See contrib(1) after installation for details.