Add explainer for encoded frame creation

create-encoded-explainer.md

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+# Explainer - Create functions for encoded data
+
+## Problem to be solved
+
+In some use cases, we need to move encoded data beyond the confines of a single PeerConnection.
+
+The use cases include:
+
+* Relaying incoming encoded data to one or more other PeerConnections as outgoing data
+* Sending data that has already been encoded (by a camera, from a file, or other sources)
+* Receiving data in encoded form from a non-WebRTC source and processing it using WebRTC
+
+Addressing these cases in the webrtc-encoded-transform API requires the ability to:
+
+* Create encoded frames
+* Clone encoded frames
+* Manipulate encoded frame metadata to conform with the requirements of its destination
+
+These are not the only functions that are needed to handle the use cases, but this explainer
+focuses on the frame creation, cloning and manipulation functions.
+
+## Approach
+
+The approach proposed as a minimum viable API consists of 3 functions:
+
+* A constructor that takes encoded data + metadata and creates an encoded frame
+* A setter that is able to modify some of the values returned from the existing GetMetadata method
+* A clone operator that takes an existing encoded frame and creates a new, independent encoded frame
+
+The clone operator may be thought of as a convenient shorthand for the constructor:
+
+```
+frame.clone = constructor(frame.getMetadata(), frame.data)
+
+```
+