Add doc in README.md : how to flash using gdb

README.md

@@ -102,6 +102,7 @@ $ make -j pinetime-app
 ```  
 
 ## How to program
+### Using make
 
  * Erase
  
@@ -128,9 +129,56 @@ $ make FLASH_MERGED_pinetime-app
 $ make help
 ```
 
+### Using GDB and a Black Magic Probe (BMP)
+You can use this procedure if you want to flash the firmware (from a release, for example) without having to build the project locally.
 
+Enter the following command into GDB:
 
- 
+```
+target extended-remote /dev/ttyACM0
+monitor swdp_scan
+attach 1
+file ./pinetime-app-full.hex 
+load
+run
+```
+
+*Note* : Sometimes, I need to run this procedure 2 times : the first time, the screen stays blank, and the second time, it works. This is something I'll need to debug...
+
+Example:
+```
+$ /home/jf/nrf52/gcc-arm-none-eabi-8-2019-q3-update/bin/arm-none-eabi-gdb
+
+(gdb) target extended-remote /dev/ttyACM0
+Remote debugging using /dev/ttyACM0
+(gdb) monitor swdp_scan
+Target voltage: ABSENT!
+Available Targets:
+No. Att Driver
+ 1      Nordic nRF52 M3/M4
+ 2      Nordic nRF52 Access Port 
+
+(gdb) attach 1
+Attaching to Remote target
+warning: No executable has been specified and target does not support
+determining executable automatically.  Try using the "file" command.
+0xfffffffe in ?? ()
+(gdb) file ./pinetime-app-full.hex 
+A program is being debugged already.
+Are you sure you want to change the file? (y or n) y
+Reading symbols from ./pinetime-app-full.hex...
+(No debugging symbols found in ./pinetime-app-full.hex)
+(gdb) load
+Loading section .sec1, size 0xb00 lma 0x0
+Loading section .sec2, size 0xf000 lma 0x1000
+Loading section .sec3, size 0x10000 lma 0x10000
+Loading section .sec4, size 0x5150 lma 0x20000
+Loading section .sec5, size 0xa000 lma 0x26000
+Loading section .sec6, size 0x10000 lma 0x30000
+Loading section .sec7, size 0xdf08 lma 0x40000
+Start address 0x0, load size 314200
+Transfer rate: 45 KB/sec, 969 bytes/write.
+```
 
 ## RTT