Update README.md

Update usage.

README.md

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 # ProtobufInspector
 Protobuf Data Parser in Fiddler
+
+## Files
+Protobuf2Json.dll
+Inspectors/ProtobufInspector.dll
+
+## Usage:
+Copy two released files into Fiddler installation folder.
+
+Once the extension is loaded by Fiddler, it will output a log as follows:
+![Loading Log](https://github.com/BlueMatthew/ProtobufInspector/raw/master/docs/res/LogOnLoading.png)
+
+## How to parse the protobuf data:
+1. If Content-Type in response headers is "application/x-protobuf", the extension will regard it as "Protobuf" data.
+2. The token value of "messageType" in "Content-Type" will be regarded as message type name. 
+3. The token value of "desc" or "Desc" in "Content-Type" will be regarded as url of descriptorset of the protos.
+4. If message type or descriptorset url doesn't exist, the message type name and local proto folder can be provides in Protobuf Inspector View manually and then try to parse the data.