cortexm_common_ldscript: add a README.md
cortexm_common_ldscript: add a README.md

Explain the test and the output you should get.

yogo1212/RIOT,aeneby/RIOT,gebart/RIOT,kbumsik/RIOT,miri64/RIOT,kYc0o/RIOT,A-Paul/RIOT,kYc0o/RIOT,kaspar030/RIOT,smlng/RIOT,basilfx/RIOT,BytesGalore/RIOT,toonst/RIOT,miri64/RIOT,josephnoir/RIOT,lazytech-org/RIOT,x3ro/RIOT,kYc0o/RIOT,avmelnikoff/RIOT,authmillenon/RIOT,avmelnikoff/RIOT,authmillenon/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,rfuentess/RIOT,miri64/RIOT,kaspar030/RIOT,cladmi/RIOT,mfrey/RIOT,mtausig/RIOT,smlng/RIOT,yogo1212/RIOT,x3ro/RIOT,mtausig/RIOT,OTAkeys/RIOT,smlng/RIOT,biboc/RIOT,OTAkeys/RIOT,mfrey/RIOT,OlegHahm/RIOT,yogo1212/RIOT,ant9000/RIOT,mtausig/RIOT,A-Paul/RIOT,BytesGalore/RIOT,authmillenon/RIOT,smlng/RIOT,authmillenon/RIOT,biboc/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,basilfx/RIOT,mtausig/RIOT,OlegHahm/RIOT,A-Paul/RIOT,gebart/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,biboc/RIOT,aeneby/RIOT,kbumsik/RIOT,aeneby/RIOT,OTAkeys/RIOT,A-Paul/RIOT,BytesGalore/RIOT,kbumsik/RIOT,ant9000/RIOT,OlegHahm/RIOT,basilfx/RIOT,RIOT-OS/RIOT,avmelnikoff/RIOT,RIOT-OS/RIOT,aeneby/RIOT,OlegHahm/RIOT,rfuentess/RIOT,jasonatran/RIOT,cladmi/RIOT,cladmi/RIOT,miri64/RIOT,BytesGalore/RIOT,gebart/RIOT,OTAkeys/RIOT,cladmi/RIOT,x3ro/RIOT,toonst/RIOT,rfuentess/RIOT,mfrey/RIOT,x3ro/RIOT,lazytech-org/RIOT,cladmi/RIOT,mfrey/RIOT,yogo1212/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kYc0o/RIOT,toonst/RIOT,yogo1212/RIOT,authmillenon/RIOT,biboc/RIOT,toonst/RIOT,basilfx/RIOT,BytesGalore/RIOT,jasonatran/RIOT,avmelnikoff/RIOT,toonst/RIOT,kaspar030/RIOT,jasonatran/RIOT,josephnoir/RIOT,ant9000/RIOT,ant9000/RIOT,A-Paul/RIOT,smlng/RIOT,x3ro/RIOT,kbumsik/RIOT,josephnoir/RIOT,avmelnikoff/RIOT,ant9000/RIOT,lazytech-org/RIOT,authmillenon/RIOT,mfrey/RIOT,gebart/RIOT,lazytech-org/RIOT,aeneby/RIOT,mtausig/RIOT,rfuentess/RIOT,basilfx/RIOT,biboc/RIOT,lazytech-org/RIOT,kbumsik/RIOT,kaspar030/RIOT,OlegHahm/RIOT,gebart/RIOT,kaspar030/RIOT