use cmake for configuration and reorganize README to markdown format

[axiomwang]

Add cmake configuration and build target so that other cmake projects can easily find this project.

[bonachea]

GASNet has always used an automake/autoconf build system to build libgasnet, and that will never change.
CMake users can build GASNet using the existing build system, and integrate the result into their CMake project using a CMake Find module, such as FindGASNet.cmake

The proposed rewrite of README loses significant technical information and is rejected for that reason.