Updated README with global state and input message

README.md

@@ -12,6 +12,54 @@ The Pricecaster backend can be configured with any class implementing **IPriceFe
 
 ![PRICECASTER](https://user-images.githubusercontent.com/4740613/136037362-bed34a49-6b83-42e1-821d-1df3d9a41477.png)
 
+
+## Data Format
+
+### Input Message
+
+The TEAL contract expects a fixed-length message consisting of:
+
+```
+  Field size
+  9           header      Literal "PRICEDATA"
+  1           version     int8 (Must be 1)
+  8           dest        This appId 
+  16          symbol      String padded with spaces e.g ("ALGO/USD        ")
+  8           price       Price. 64bit integer.
+  8           priceexp    Price exponent. Interpret as two-compliment, Big-Endian 64bit
+  8           conf        Confidence (stdev). 64bit integer. 
+  8           slot        Valid-slot of this aggregate price.
+  8           ts          timestamp of this price submitted by PriceFetcher service
+  32          s           Signature s-component
+  32          r           Signature r-component 
+
+  Size: 138 bytes. 
+```
+
+### Global state
+
+The global state that is mantained by the contract consists of the following fields:
+
+```
+sym      : byte[] Symbol to keep price for   
+vaddr    : byte[] Validator account          
+price    : uint64 current price 
+stdev    : uint64 current confidence (standard deviation)
+slot     : uint64 slot of this onchain publication
+exp      : byte[] exponent. Interpret as two-compliment, Big-Endian 64bit
+ts       : uint64 last timestamp
+```
+
+#### Price parsing
+
+The exponent is stored as a byte array containing a signed, two-complement 64-bit Big-Endian integer, as some networks like Pyth publish negative values here. For example, to parse the byte array from JS:
+
+```
+    const stExp = await tools.readAppGlobalStateByKey(algodClient, appId, VALIDATOR_ADDR, 'exp')
+    const bufExp = Buffer.from(stExp, 'base64')
+    const val = bufExp.readBigInt64BE()
+```
+
 ## Backend Configuration
 
 The backend will read configuration from a `settings.ts` file pointed by the `PRICECASTER_SETTINGS` environment variable.