feat(ard): Add adr 002 initial draft

adr/adr-002-jan-replicants.md

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+# ADR #002: {Jan replicant - Composable plugin}
+
+## Changelog
+- Oct 4th 2023: Initial draft
+
+## Authors
+- @vuonghoainam - Hiro
+- @louis-jan
+
+## Status
+Proposed
+
+## Context
+
+### Business context
+Jan can be a platform and let builders build their own `replicants` using existing tools
+- Use-case 1: Medical AI startup uploads "case notes" to Jan, wants to ask it questions (i.e. medical audit)
+- Use-case 2: Legal e-discovery: very large amount of documents (~10-15k pages) are uploaded, data is very private and cannot be leaked
+- Use-case 3: Jan wants to use Jan to have a QnA chatbot to answer questions on docs
+- Use-case 4: Jan wants to use Jan to have a codellama RAG on its own codebase, to generate new PRs
+
+### Extra context
+- There are many use cases that the community can develop and sell to the users through Jan as plugin. Jan needs to streamline higher value chain.
+- This brings more value and more option to all kind of user
+- This can help building ecosystem and streamline value end to end (Jan,  plugins/ model creators, Jan users - enterprise/ individual)
+- We at Jan cannot build plugins more on our own, but this one should serve as featured example like [OpenAI Retrieval plugin](https://github.com/openai/chatgpt-retrieval-plugin) does.
+- [#232](https://github.com/janhq/jan/issues/232)
+
+## Decision
+
+- User can browse and install plugins (with recommended model - llama2, claude, openai …) - This requires plugin dependencies.
+- Jan provide consistent interface for plugin developer to use:
+    - Use LLM (this can be switched in runtime) - i.e Dev in llama2-7b but user can use with llama2-70b. Can choose another model as well
+    - Plugin can have API for CRUD indices in vectorDB/ DB, and Jan only exposes corresponding data to the app
+    - A place for a plugin to store the files for persistence
+- This works seamlessly on desktop/ Jan hosted version with Jan API abstraction.
+
+### Simple UX
+![UX](images/adr-002-01.png "UX")
+
+### Component design
+![Component design](images/adr-002-02.png "Component design")
+
+## Consequences
+- Jan user can build their own replicants (and buy from others too) in an easy way
+- Clear design for plugin and Jan platform development
+
+## Reference