Add some high level notes about rust-miniscript design

doc/high_level_design.md

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+## Miniscript High level design
+
+![alt text](./miniscript_high_level_arch.png)
+
+As of 10.0.0 rust-miniscript is roughly divided into following components:
+
+* **Miniscript Core**(<span style="color:green">Green box</span>): This is the core of the library. It contains the core data structures and algorithms for miniscript and output descriptors.
+All the users are expected to interact with the library using the descriptor APIs. Raw miniscript should be used only if you know what you are doing.
+* **Policy**(<span style="color:red">Red box</span>): This module consists of _two_ separate policy languages with different purposes:
+    - _Concrete Policy_: Concrete policy is a simple language which allows users to specify the policy for a descriptor. The miniscript compiler compiles this concrete policy into a descriptor.
+    - _Semantic Policy_: The policy module also provides a "semantic policy" for analysis of descriptors and miniscript. Unlike the concrete policy, which is implemented and used by other projects, the semantic policy is purely part of rust-miniscript. The APIs on this object can be used to analyze descriptors and miniscripts for various properties.
+
+* **Psbt**(<span style="color:blue">Blue box</span>): Psbt support for descriptors. Provides several new extension APIs to interact for miniscript descriptors in a psbt including finalizer, planning, signing, descriptor updates, etc.
+* **Interpreter**(<span style="color:yellow">Yellow box</span>): Script Interpreter for miniscript. It also provides APIs to figure out the satisfied constraints for a given transaction input. Useful for detecting which spend path the transaction output was spend with, as in the case of watchtowers or general monitoring, block explorers, etc.