Add a contexts/README.md to explain the DID context versioning scheme.

contexts/README.md

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+# DID Contexts
+
+## Background
+
+JSON-LD Context versioning (and schema versioning in general) is a complex 
+topic, involving the need to balance security and developer convenience.
+
+On one end of the spectrum, some use cases require cryptographically bound and 
+immutable contexts, and will need to use a contest-addressable URL mechanism,
+such as the in-progress 
+[Hashlink](https://tools.ietf.org/html/draft-sporny-hashlink-03) spec.
+
+On the opposite end of the spectrum, developers that can tolerate the fact that
+the DID context is still changing (in the Credentials Community Group and the 
+forthcoming DID Working Group), may want a way to just say "I want to use the
+latest in-progress `@context`", without constraining the versions.
+
+And between the two extremes, some developers may want to use strongly versioned
+contexts (without necessarily cryptographically binding assurance).
+
+## DID Context Versions
+
+The current system for DID context versioning is:
+
+1. Use `https://www.w3.org/2019/did/v1` as an alias to the latest version of the
+spec. Once the DID Working Group work concludes, the final v1 version will be
+frozen and made immutable.
+2. Use intermediate pre-v1 versioned URLs such as `https://w3id.org/did/v0.11`
+if you need to refer to a specific context version.
+3. Use a content-addressable mechanism such as Hashlinks in addition to the
+versioned URLs if your use case requires it.
+
+Contexts will be versioned according to a modified Semantic Versioning scheme.
+
+Before v1.0 (minted at the end of the Working Group process), **all changes to
+the context will be marked as BREAKING changes**, by incrementing the minor
+`v0.x` version, with no patch component.
+
+## Changelog
+
+### v0.11 - `https://w3id.org/did/v0.11`
+
+Start of changelog