add readme for token example actor

Author: alexytsu

testing/integration/actors/fil_token_actor/README.md

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+# FIL Token Actor
+
+This creates a FIP-??? compliant fungible token that wraps the value of of
+native FIL.
+
+Calling the `mint` (non-standard) method on this actor will increase the
+caller's WFIL balance by the amount of FIL sent in the message. The WFIL can be
+transferred between any FIP-??? compliant wallets (which have `token_received`
+hook) by either direct transfer or delegated transfer.
+
+https://etherscan.io/tx/0x96a7155b44b77c173e7c534ae1ceca536ba2ce534012ff844cf8c1737bc54921
+
+## Direct transfer flow
+
+Call `transfer(TransferParams)` to directly transfer tokens from the caller's
+wallet to a receiving address.
+
+## Delegated transfer flow
+
+1. Call `increase_allowance(AllowanceParams)` to increase the "spenders"
+   allowance
+2. The "spender" can then call this actor with
+   `transfer_from(TransferFromParams)` to
+
+## Transferring tokens to this actor
+
+Transferring WFIL to the address of this actor itself will result in the WFIL
+being burnt and the corresponding FIL being credited back. This prevents the
+case where tokens are unintentionally locked in to contracts that are unable to
+receive them. This flow requires the actor to implement its own `token_received`
+hook.
+
+However, also compliant with the token standard would be for this actor to omit
+the `token_received` hook. In this case, transfers to the contract itself would
+simply be rejected, which also prevents unintentional loss of tokens.