Add Installation section and MELPA badges

Author: syohex

README.md

@@ -1,7 +1,20 @@
-# helm-orgcard.el
+# helm-orgcard.el [![melpa badge][melpa-badge]][melpa-link] [![melpa stable badge][melpa-stable-badge]][melpa-stable-link]
 
 Browse the orgcard by helm.
 
 ## Screenshot
 
 ![helm-orgcard](image/helm-orgcard.png)
+
+## Installation
+
+`helm-orgcard` is available on [MELPA][melpa-link] and [MELPA-STABLE][melpa-stable-link].
+
+You can install `helm-orgcard` with the following command.
+
+<kbd>M-x package-install [RET] helm-orgcard [RET]</kbd>
+
+[melpa-link]: https://melpa.org/#/helm-orgcard
+[melpa-stable-link]: https://stable.melpa.org/#/helm-orgcard
+[melpa-badge]: https://melpa.org/packages/helm-orgcard-badge.svg
+[melpa-stable-badge]: https://stable.melpa.org/packages/helm-orgcard-badge.svg