add ADR002 to document the storage

doc/adr002-storage.md

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+# ADR002: Storage Layout
+
+**Author:** Igor Konnov, 2024
+
+This ADR discusses the simple layout of the Soroban transactions that are
+fetched from a Stellar network.
+
+Our goals are:
+
+ - Store contract calls retrieved from the Horizon API, see [storage.ts][].
+ - Access calls indendently for verification.
+ - Tag calls as `stored`, `verified`, `failed`, etc.
+
+To keep things simple in the activation stage, we are simply using the
+filesystem instead of a fully-featured database. This is sufficient to
+demonstrate our idea.  When the users expect a storage that its durable,
+consistent, fail-free, etc., we would have to implement the storage with a solid
+database library.
+
+In the rest of this document, we refer to the root of the storage directory as
+`${STOR}`. We expect the user to provide the Solarkraft commands with the
+root directory. By default, `${STOR}` would point to
+`$HOME/.solarkraft/.stor`.
+
+## 1. Storing extracted calls
+
+We have the following requirements to the storage:
+
+ - It should support contract calls (from transactions) that are extracted for
+   different contracts.
+
+ - It should be relatively easy to order these transactions, e.g., by ledger height.
+
+ - Transactions that happen in the same block should not collide.
+
+
+Given all these requirements, we store every contract call in its own file called:
+
+```
+${STOR}/${contractId}/${height}/entry-${txHash}.json
+```
+
+The values `${contractId}`, `${height}`, `${txHash}` are the values of the
+fields that are defined in [storage.ts][].
+
+The file is a JSON serialization of [ContractCallEntry][]. Some care is required
+to serialize big integers, we use [json-bigint][]. Additionally, we serialized
+`OrderedMap` as an array.
+
+## 2. Storing verification results
+
+The verification results are to be stored in a file called:
+
+```
+${STOR}/${contractId}/${height}/verification-${txHash}.json
+```
+
+The exact format of this file is to be defined later.
+
+
+
+[storage.ts]: https://github.com/freespek/solarkraft/blob/main/solarkraft/src/fetcher/storage.ts
+[ContractCallEntry]: https://github.com/freespek/solarkraft/blob/38d57fd51082feab9215a77c555bdccdc961aa26/solarkraft/src/fetcher/storage.ts#L23
+[json-bigint]: https://www.npmjs.com/package/@types/json-bigint