cortexm_common_ldscript: add a README.md

Explain the test and the output you should get.

tests/cortexm_common_ldscript/README.md

+Cortexm-common ldscript
+=======================
+
+This test checks the support for building firmwares with a rom offset and
+specific sized firmwares using `cortexm-common` ldscript.
+
+When building `all`, the `compile-tests` target is also build and executes build
+time compilation tests checking different firmwares configurations verified by
+inspecting the result elf files.
+
+
+Test output
+-----------
+
+For a board that does not have a `ROM_OFFSET` variable configured by default
+(== a board without bootloader installed), it produces the following output
+after the normal compilation:
+
+```
+make all BOARD=iotlab-m3
+...
+Test rom offset 1 byte overflow detection: [OK]
+Test rom offset substracted from rom length in elffile: [SKIP](Reason: board does not have a ROM_OFFSET configured)
+Test compilation with offset 0x1000: [OK]
+Test compilation with offset 0x2000: [OK]
+Test compilation with half ROM length: [OK]
+Test ROM overflow detection (too_big_for_rom): [OK]
+Test ROM overflow detection (offset_and_romlen): [OK]
+```
+
+For a bord that have a `ROM_OFFSET` variable configured by default (== a board
+with bootloader installed), it produces the following output after the normal
+compilation:
+
+```
+make BOARD=bluepill  PROGRAMMER=dfu-util
+...
+Test rom offset 1 byte overflow detection: [OK]
+Test rom offset substracted from rom length in elffile: [OK]
+Test compilation with offset 0x1000: [OK]
+Test compilation with offset 0x2000: [OK]
+Test compilation with half ROM length: [OK]
+Test ROM overflow detection (too_big_for_rom): [OK]
+Test ROM overflow detection (offset_and_romlen): [OK]
+```