* add Jordan Williams to both CONTRIBUTORS and AUTHORS

* alias benchmark libraries

Provide aliased CMake targets for the benchmark and benchmark_main targets.
The alias targets are namespaced under benchmark::, which is the namespace when they are exported.
I chose not to use either the PROJECT_NAME or the namespace variable but to hard-code the namespace.
This is because the benchmark and benchmark_main targets are hard-coded by name themselves.
Hard-coding the namespace is also much cleaner and easier to read.

* link to aliased benchmark targets

It is safer to link against namespaced targets because of how CMake interprets the double colon.
Typo's will be caught by CMake at configuration-time instead of during compile / link time.

* document the provided alias targets

* add "Usage with CMake" section in documentation

This section covers linking against the alias/import CMake targets and including them using either find_package or add_subdirectory.

* format the "Usage with CMake" README section

Added a newline after the "Usage with CMake" section header.
Dropped the header level of the section by one to make it a direct subsection of the "Usage" section.
Wrapped lines to be no longer than 80 characters in length.