docs: Update README with install instructions

Adds a code snippet to the README demonstrating how to install the Simplenote plugin using Package Control. This makes it easier for users to set up the plugin.

README.md

@@ -1,7 +1,8 @@
 # Simplenote for Sublime Text
 ========================
+
 Sublime Text 3 & 4 plugin of Simplenote, Use Sublime Text as a Simplenote client.
---------
+
 ```sh
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