doc: Add a simple DESIGN document

Describe objects and how they interact with each other.

DESIGN

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+Some simple design notes to guide people
+
+- MGCPGrammar can parse requests and replies
+
+- MGCPCommand base class of commands
+   - asDatagram to create message
+
+- MGCPTrunk a trunk with a destination and nr. endpoints
+   - MGCPVirtualTrunk for 1@mgw.. in hex
+   - MGCPE1Trunk for ds-1/... handling
+   - both hold MGCPEndpoints
+
+- MGCPTransaction a command to create a transaction
+   - callback for completion (wth response)
+   - callback for a timeout
+   - created with an endpoint and the callagent
+
+- MGCPCallAgent serves the Call-Agent port
+   - Generates the transaction id for a transaction
+   - Does the retransmission
+   - Handles timeout
+   - Recycles transaction after three minutes
+     (to block a transaction identifier)
+
+