Linux: Add usage steps for PRU firmware to Readme

Tools/Linux_HAL_Essentials/testpru/README.md

@@ -1,4 +1,32 @@
-testpru
+PRU PWM
 =======
 
-PRU PWM
+###Updating kernel
+* Check your kernel version using `uname -r`
+* If you get 3.8.13bone56+ as output then you may skip Updating kernel.
+* For other kernel versions write following cmds in your BBB:
+```
+wget https://rcn-ee.net/deb/wheezy-armhf/v3.8.13-bone57/install-me.sh
+chmod +x install-me.sh
+./install-me.sh
+```
+* After above steps reboot and check your kernel version using `uname -r`,it should be `3.8.13bone57`
+
+###Setting Up PRU Compiler
+* Download [PRU C Compiler v2.0.0B2 installer](http://software-dl.ti.com/codegen/non-esd/downloads/beta.htm)
+* Please check the path where you install PRU compiler.
+* Setting environment variable
+```
+echo export PATH=/path/to/pru/compiler/bin:$PATH > ~/.bashrc
+echo export PRU_C_DIR="/path/to/pru/compiler/include;/path/to/pru/compiler/lib" > ~/.bashrc
+source ~/.bashrc
+```
+
+*note: semicolons in second command were intended*
+
+###Compiling and loading the code
+* just `make`
+* copy generated executable `testpru1` to `ardupilot/Tools/Linux_HAL_Essentials/`.
+* To load firmware use 
+  - `./startup.sh load` : only once after reboot it will copy overlays and firmware to `/lib/firmware/` and loads them.
+  - `./startup.sh reload` : it only copies firmware to `/lib/firmware/` and reloads pru firmware.