adr-002 lite client

docs/architecture/adr-002-light-client-adr-index.md

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+# ADR 002: Light Client ADR Index
+
+## Changelog
+
+- 2019-10-30: First draft.
+- 2019-12-27: Add diagram and align with the code.
+- 2020-01-22: Refactor into multiple ADRs
+
+## Status
+
+ADR Index.
+
+## Context
+
+This is the first of multiple ADRs about the Tendermint light client in Rust.
+Light client specification and implementation is an ongoing work in progress, in 
+[English][english-spec],
+[TLA+][tla-spec], 
+[Go][go-impl],
+and [Rust][rust-impl]. Note/TODO these links will soon be stale and need
+updating.
+
+It is especially important to review the Motivation and Structure of the Light
+Client as described in the English Specification before reading these ADRs for
+the necessary context.
+
+Here we are concerned primarily with the implementation of a light node in Rust.
+The light node process makes RPC requests to full nodes for blockchain data, uses
+the core verification library to verify this data, and updates its state accordingly. 
+It also makes requests to other full nodes to detect and report on forks.
+
+That said, as much as possible, we would like the implementation here to be
+re-usable for the IBC protocol, which supports communiction between blockchains. 
+In this case, instead of making RPC requests, IBC-enabled blockchains receive the relevant data in transactions and
+verify it using the same core verification library. Thus implementations should
+abstract over the source of data as necessary.
+For more information on IBC, see the 
+[spec repo](https://github.com/cosmos/ics),
+especially the specifications for
+[client
+semantics](https://github.com/cosmos/ics/tree/master/spec/ics-002-client-semantics)
+and [handler
+interface](https://github.com/cosmos/ics/tree/master/spec/ics-025-handler-interface)
+
+A complete implementation of the light node includes many components, each of
+which deserves their own ADR. This ADR thus serves as an index for those, and
+may be updated over time as other components come into view or
+existing components are iterated upon.
+
+Components include (TODO: links):
+
+- ADR-003 - Core Verification Library: Data types, traits, and public API for
+  the core verification protocol, with support for use in light nodes and IBC
+  implemented
+- ADR-004 - Command Line Interface: Choice of framework for the CLI and
+  daemon, including config, arguments, logging, errors, etc.
+- ADR-005 - Fork Detection Module: Simple module to manage an address book of 
+  peers and search for conflicting commits
+
+
+A schematic diagram of the light node, as taken from the [English
+specification][english-spec], is provided below:
+
+![Light Node Diagram](assets/light-node.png).
+
+
+[english-spec]: https://github.com/tendermint/spec/tree/bucky/light-reorg/spec/consensus/light
+[tla-spec]: https://github.com/interchainio/verification/tree/igor/lite/spec/light-client
+[go-impl]: https://github.com/tendermint/tendermint/tree/master/lite2
+[rust-impl]: https://github.com/interchainio/tendermint-rs/tree/master/tendermint-lite