Adding small README describing what bin2ota.py does and how its used

Author: aentinger

extras/tools/README.md

@@ -0,0 +1,26 @@
+`bin2ota.py`
+============
+This tool can be used to extend (actually prefix) a binary generated with e.g. the Arduino IDE with the required length and crc values required to perform an OTA (Over-The-Air) update of the firmware.
+
+### How-To-Use
+```bash
+./bin2ota.py sketch.bin sketch-ota.bin
+```
+#### `sketch.bin`
+```bash
+0000000   8000 2000 749D 0000 7485 0000 7485 0000
+0000010   0000 0000 0000 0000 0000 0000 0000 0000
+0000020   0000 0000 0000 0000 0000 0000 7485 0000
+0000030   0000 0000 0000 0000 7485 0000 74F1 0000
+...
+```
+* `length(sketch.bin) = 0x0003'A5E0`
+* `CRC32(sketch.bin)  = 0xA9D1'265B`
+
+#### `sketch-ota.bin`
+```bash
+0000000   A5E0 0003 265B A9D1 8000 2000 749D 0000
+0000010   7485 0000 7485 0000 0000 0000 0000 0000
+0000020   0000 0000 0000 0000 0000 0000 0000 0000
+0000030   0000 0000 7485 0000 0000 0000 0000 0000
+...