Add README

Put available features.

README.md

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+# mio-ai
+
+[![Mio](./docs/assets/91986900_p7.jpg)](https://www.pixiv.net/en/artworks/91986900)
+
+Many-in-one AI client.
+
+## Supported Platforms and Features
+
+* OpenAI
+  - [X] models
+  - [X] completions
+  - [X] chat
+  - [X] edits
+  - [X] images
+      - [X] generate
+      - [X] edit
+      - [X] variation
+  - [ ] embeddings
+  - [ ] audio
+  - [ ] files
+  - [ ] fine-tunes
+  - [ ] moderations