Added docs about WIRINGPI_CODES environment variable

Author: Andrew Menagarishvili

README.md

@@ -60,7 +60,7 @@ a pin based GPIO access library written in C.
 You can grab the latest version of PowerShell IoT by running:
 
 ```powershell
-sudo pwsh
+sudo WIRINGPI_CODES=1 pwsh
 Install-Module Microsoft.PowerShell.IoT
 ```
 
@@ -133,9 +133,14 @@ _NOTE: If you'd rather not use the script, simply copy the `out/Microsoft.PowerS
 First, you must run pwsh with sudo:
 
 ```powershell
-sudo pwsh
+sudo WIRINGPI_CODES=1 pwsh
 ```
 
+##### About `WIRINGPI_CODES` environment variable
+
+`Microsoft.PowerShell.IoT` module internally uses [WiringPi library](http://wiringpi.com/reference/setup) which has a default behavior of terminating current process (in this case - PowerShell) even on non-critical errors in setup functions.
+To avoid such crashes define `WIRINGPI_CODES` environment variable either when starting PowerShell (see example above) or through configuration scripts - example for an interactive login shell - `echo "export WIRINGPI_CODES=1"|sudo tee -a /etc/profile.d/WiringPiCodes.sh`
+
 If you have the `Microsoft.PowerShell.IoT` module in your PSModulePath:
 
 ```powershell