Adding gitignore plugin README

plugins/gitignore/README.md

+# gitignore
+
+This plugin enables you the use of 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 displays a list of all of the currently support gitignore.io templates.
+* `gi [TEMPLATENAME]` Show 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.