Added README.md file for atom plugin

plugins/atom/README.md

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+## atom
+
+Plugin for Atom, a cross platform text and code editor, available for Linux, Mac OS X, and Windows.
+
+### Requirements
+
+ * [Atom](https://atom.io/)
+
+### Usage
+
+ * If `at` command is called without an argument, launch Atom
+
+ * If `at` is passed a directory, `cd` to it and open it in Atom
+
+ * If `at` is passed a file, open it in Atom
+
+ * if `att` command is called, it is equivalent to `at .`, opening the current folder in Atom