Update README with challenge instructions

Author: gballet

README.md

@@ -1,2 +1,57 @@
-# wrc20-examples
+# WRC-20 challenge examples
+
 This repository contains examples of WRC20 tokens written in different languages.
+
+## Joining the challenge
+
+Have a look at [this gist](https://gist.github.com/axic/16158c5c88fbc7b1d09dfa8c658bc363), implement the logic for the following contract (pseudocode) in your favorite language:
+
+```javascript
+// main ewasm entry point
+function main() { 
+  if (eei_calldatasize() < 4)
+    eei_revert(0, 0)
+  let selector = eei_calldatacopy(0, 4)
+  switch selector {
+    case 0x9993021a:
+      do_balance()
+    case 0x5d359fbd:
+      do_transfer()
+    default:
+      eei_revert(0, 0)
+  }
+}
+
+function do_balance() {
+  if (eei_calldatasize() != 24)
+    eei_revert(0, 0)
+
+  let address = eei_calldatacopy(4, 20)
+  // make sure that address is 160 bits here,
+  // but storage key is 256 bits so need to pad it somehow
+  let balance = eei_storageload(address)
+  eei_return(balance)
+}
+
+function do_transfer() {
+  if (eei_calldatasize() != 32)
+    eei_revert(0, 0)
+
+  let sender = eei_sender()
+  let recipient = eei_calldatacopy(4, 20)
+  let value = eei_calldatacopy(24, 8)
+  let sender_balance = eei_storageload(sender)
+  let recipient_balance = eei_storageload(recipient)
+  
+  if (sender_balance < value)
+    eei_revert(0, 0)
+
+  sender_balance -= value
+  recipient_balance += value
+
+  eei_storagestore(sender, sender_balance)
+  eei_storagestore(recipient, recipient_balance)
+}
+```
+
+Then simply put it in a directory named after your language and send us a PR.