Add .env to gitignore for dotenv files

Author: mikekistler

Dotnet.gitignore

@@ -36,6 +36,9 @@ ScaffoldingReadMe.txt
 # NuGet Symbol Packages
 *.snupkg
 
+# dotenv environment variables file
+.env*
+
 # Others
 ~$*
 *~
@@ -51,4 +54,4 @@ CodeCoverage/
 # NUnit
 *.VisualState.xml
 TestResult.xml
-nunit-*.xml
+nunit-*.xml