gitignore: add README (#7312)

plugins/gitignore/README.md

+# gitignore
+
+This plugin enables you the use of [gitignore.io](https://www.gitignore.io/) from the command line. You need an active internet connection.
+
+To use it, add `gitignore` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... gitignore)
+```
+
+## Plugin commands
+
+* `gi list`: List all the currently supported gitignore.io templates.
+
+* `gi [TEMPLATENAME]`: Show git-ignore output on the command line, e.g. `gi java` to exclude class and package files.
+
+* `gi [TEMPLATENAME] >> .gitignore`: Appending programming language settings to your projects .gitignore.