Add expoter README

Includes table of exporter supported toolchains by platform.
Merge pull request #104 from stephenpaulger/exporter-readme

Add exporter README
Added configuration mechanism

If the build system finds a file named mbed_config.h, it will automatically
define a HAVE_MBED_CONFIG_H macro. Various libraries/components can use
this mechanism to provide compile-time configuration for user projects.
Merge remote-tracking branch 'origin/master'