Short note on environment variable support [Skip CI]

Author: nblumhardt

README.md

@@ -80,3 +80,14 @@ This is useful in ASP.NET Core applications, which will often specify minimum le
         }
     }
 ```
+
+### Environment variables
+
+If your application enables the environment variable configuration source (`AddEnvironmentVariables()`) you can add or override Serilog configuration through the environment.
+
+For example, to set the minimum log level using the `Windows` command prompt:
+
+```
+set Serilog:MinimumLevel=Debug
+dotnet run
+```