add asserter validation doc (coinbase#328)

asserter/README.md

@@ -8,10 +8,62 @@ populated, fields are in the correct format, and transaction operations only
 contain types and statuses specified in the /network/status endpoint.
 
 If you want more intensive validation, try running the
-[Rosetta Validator](https://github.com/coinbase/rosetta-validator).
+[Rosetta CLI](https://github.com/coinbase/rosetta-cli).
 
 ## Installation
 
 ```shell
 go get github.com/coinbase/rosetta-sdk-go/asserter
 ```
+
+## Validation file
+Asserter package also allows you to specify a validation file which can be used to have a
+stricter validation against your implementation.
+
+A simple validation files looks like [this](./data/validation_fee_and_payment_balanced.json).
+Let's break it down and see what it means
+
+```
+"enabled": true
+```
+Specifies if we want to enable this validation or not.
+
+```
+"chain_type": "account",
+```
+Specify the chain type. Supported types are `account` and `utxo`. Right now we only support `account` based implementation for validation using this file.
+
+```
+"payment": {
+  "name": "PAYMENT",
+  "operation": {
+    "count": 2,
+    "should_balance": true
+  }
+},
+```
+
+This first validation will validate payment or transaction type. The `payment` object is defined below
+
+* name: The string used for defining a payment or transaction
+* operation:
+    * count: Count of payment operations (defined by `name`) which should be present in the transaction. If the number is -1, then we won't validate the count
+    * should_balance: If the sum total of the `amount` defined in the all the operations (defined by `name`) in a particular transaction should add up to 0 or not. This is really helpful to validate a debit and credit operation in the implementation
+
+```
+"fee": {
+  "name": "FEE",
+  "operation": {
+    "count": 1,
+    "should_balance": false
+  }
+}
+```
+Similar to `payment` type we have `fee` type. The fields here have the same usage as above.
+
+---
+**NOTE**
+
+Right now we only support `payment` and `fee` operation type with `count` and `total` balance match. We will keep adding more validations to it.
+
+--- 

asserter/asserter.go

@@ -42,6 +42,9 @@ type Asserter struct {
 	validations             *Validations
 }
 
+// Validations is used to define stricter validations
+// on the transaction. Fore more details please refer to
+// https://github.com/coinbase/rosetta-sdk-go/tree/master/asserter#readme
 type Validations struct {
 	Enabled   bool                 `json:"enabled"`
 	ChainType ChainType            `json:"chain_type"`