Git ignore pattern for Yarn and JetBrains products (#40)

Before there were only ignore pattern for the Node.js `node_modules`
folder, but specific pattern for Yarn [1] were not present.
Because the fantastic JetBrains products [2] like GoLand [3] (or
respectively IntelliJ [4] with the official Go plugin [5]) are an
integral part of my daily toolbox the pattern have also been added.

[1]: https://yarnpkg.com
[2]: https://www.jetbrains.com
[3]: https://www.jetbrains.com/go
[4]: https://www.jetbrains.com
[5]: https://plugins.jetbrains.com

Closes GH-39

.gitignore

@@ -4,7 +4,19 @@
 # Configuration to specify paths to ignore.
 # See https://git-scm.com/docs/gitignore for more details.
 
+# +--------------------+
+# + JetBrains Products +
+# +--------------------+
+# +--- IntelliJ IDEA ---+
+.idea/*
+!.idea/runConfigurations
+!.idea/saved-exports
+!.idea/watcherTasks.xml
+
 # +---------+
 # + Node.js +
 # +---------+
 **/node_modules/
+yarn-debug.log*
+yarn-error.log*
+.yarn-integrity