add install ardupilot section

README.md

@@ -49,6 +49,7 @@ sudo apt-get update
 sudo apt-get install build-essential cmake git
 git clone https://github.com/mavlink/MAVSDK.git
 cd MAVSDK
+git checkout tags/v2.9.1
 git submodule update --init --recursive
 cmake -Bbuild/default -DCMAKE_BUILD_TYPE=Release -H.
 cmake --build build/default -j8
@@ -59,6 +60,14 @@ In case of an error "The python version is too old, expecting 3, 6, 9" during Ar
 install 3.6 python (most probably under alias python3) and
 just change 1 line in ardupilot/modules/waf/waf-light from ... python -> python3
 
+##Ardupilot
+git clone --recursive https://github.com/ArduPilot/ardupilot.git
+cd ardupilot
+./Tools/environment_install/install-prereqs-ubuntu.sh -y
+. ~/.profile
+./waf configure --board=sitl
+./waf build
+
 ## Build autopilot application
 cd src && cmake . && make