[Docs] add mono version compatiblity for Ubuntu.

Issue details

Current instructions on linux read

On Linux
Requirements
Those instructions were tested on Ubuntu 18.04. Please install Mono, either :

sudo apt install mono-runtime libmono-system-windows-forms4.0-cil libmono-system-core4.0-cil libmono-winforms4.0-cil libmono-corlib4.0-cil libmono-system-management4.0-cil libmono-system-xml-linq4.0-cil
or full Mono :

sudo apt install mono-complete
Launching
Get the lastest zipped version of Mission Planner here : https://firmware.ardupilot.org/Tools/MissionPlanner/MissionPlanner-latest.zip
Unzip in the directory you want
Go into the directory
run with mono MissionPlanner.exe

However on 18.04 this will install a very old version of mono (4.x) Current version of Mono is 6.x

On my system at least, running the old 4.x version of mono and the Latest version of Mission planner failed.

However using the latest version of mono using instructions here worked out OK.

Might be best to have the build system add a mono compatibility table to the readme.

If Mission Planner is supposed to work on mono-v4 then just go ahead and close this issue.

Thanks for this great bit of software!

Version

1.3.7563

Platform

[ ] All
[ ] AntennaTracker
[ ] Copter
[x ] Plane
[ ] Rover
[ ] Sub

Airframe type

N/A

Hardware type

N/A

Logs

Error was pretty basic... it was..

System.EntryPointNotFoundException: GetNativeSystemInfo

Essentially mono v4 was not able to retrieve the system info on Linux, but for some reason v6 worked OK.

Locate the Linux and Ubuntu Mono instructions in the README or documentation, then review the reported Mono 4.x and 6.x compatibility issue. Document the supported Mono versions or add a compatibility table, and verify that the resulting installation and launch instructions match the tested setup.