Created sdk structure in lex's rewrite (markdown)

sdk-structure-in-lex's-rewrite.md

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+refers to https://github.com/lbryio/lbry-sdk/tree/fast_wallet_server_sync branch
+
+- overall goal: unify APIs between wallet server and client
+- service dir
+    - API - the high-level interface that's shared between wallet server and client
+        - wraps Service
+        - has info that's used to generate api docs, cli docs/parsing/validation, etc.
+            - data is in [metadata.py](http://metadata.py/)
+            - [parser.py](http://parser.py/) does the generating
+    - Service - implements the API. comes in two varieties: LightClient and FullNode
+    - LightClient
+        - inherits from Service
+        - conceptually, requires a connection to another FullNode to interact with blockchain
+    - FullNode
+        - inherits from Service
+        - requires a lbrycrd instance to sync from
+- db dir
+    - Database - interface to the database
+        - instantiates db
+        - mostly calls functions in queries dir
+    - queries dir
+        - contains the actual sql queries
+        - queries written with SQLAlchemy
+    - [tables.py](http://tables.py/) contains DB table structure
+        - migrations are generated from this
+        - also used to validate queries (e.g. makes sure the columns extist, types are correct, etc)
+- blockchain dir
+    - all new code. pretty self-contained
+    - talks to lbrycrd (binary and sqlite db)
+    - Ledger
+        - similar to chainparams.cpp. defines blockchain constants
+    - sync dir - contains all the blockchain sync code
+        - has some code to make db calls. would be nice if this code lived in db dir, but it doesn't work in practice