add merchant portant registration docs

Author: rexlow

docs/merchant-portal.md

@@ -0,0 +1,15 @@
+### Register and obtain credentials from merchant portal
+
+Go to our merchant portal in [Production Merchant Portal](https://merchant.revenuemonster.my/) or [Sandbox Merchant Portal](https://sb-merchant.revenuemonster.my/) to sign up as merchant.
+
+After this, obtain `clientID` and `clientSecret` from the portal, as illustrated below.
+
+![ClientIDClientSecret](https://storage.googleapis.com/rm-portal-assets/img/rm-landing/clientIDclientSecret.png)
+
+Generate public key and private key from the portal.
+
+![PrivateKeyPublicKey](https://storage.googleapis.com/rm-portal-assets/img/rm-landing/privateKeypublicKey.PNG)
+
+Store private key for own use and public key at portal. 
+
+![PastePublicKey](https://storage.googleapis.com/rm-portal-assets/img/rm-landing/pastePublicKey.png)