Update JetBrains.gitignore

Added a forgotten IDE to the .gitignore documentation.

Why?
For completeness and correctness

What?
[Webstorm](https://www.jetbrains.com/webstorm).

Global/JetBrains.gitignore

-# Covers JetBrains IDEs: IntelliJ, RubyMine, PhpStorm, AppCode, PyCharm, CLion, Android Studio
+# Covers JetBrains IDEs: IntelliJ, RubyMine, PhpStorm, AppCode, PyCharm, CLion, Android Studio and Webstorm
 
 *.iml