Move the library files used by both nanostack and client into a common
location. This allows both client and nanostack to be built stand-alone.
The patch moves the following libraries into a common location:
-mbed-client-c
-mbed-client-randlib
-mbed-trace
-nanostack-hal-mbed-cmsis-rtos
-nanostack-libservice
-sal-stack-nanostack-eventloop
And moves the library 'coap-service' from client into nanostack.
Replacing with source code replicated from respective repositories including
mbed-client - TAG mbed-os-5.0-rc1
mbed-client-c - TAG mbed-os-5.0-rc1
mbed-client-classic - TAG mbed-os-5.0-rc2
mbed-client-mbed-tls - TAG mbed-os-5.0-rc1
mbed-client-randlib - TAG mbed-os-5.0-rc1
coap-service - TAG mbed-os-5.0-rc1
Adding CONTRIBUTING.md file explaining how to submit patches for any of the components belonging to FEATURE_CLIENT