Add README.md describing IP-Adapter model formats.

invokeai/backend/ip_adapter/README.md

@@ -0,0 +1,36 @@
+# IP-Adapter Model Formats
+
+The official IP-Adapter models are released here: [h94/IP-Adapter](https://huggingface.co/h94/IP-Adapter)
+
+This official model repo does not integrate well with InvokeAI's current approach to model management, so we have defined a new file structure for IP-Adapter models. The InvokeAI format is described below.
+
+## CLIP Vision Models
+
+CLIP Vision models are organized in `diffusers`` format. The expected directory structure is:
+
+```bash
+ip_adapter_sd_image_encoder/
+├── config.json
+└── model.safetensors
+```
+
+## IP-Adapter Models
+
+IP-Adapter models are stored in a directory containing two files
+- `image_encoder.txt`: A text file containing the model identifier for the CLIP Vision encoder that is intended to be used with this IP-Adapter model.
+- `ip_adapter.bin`: The IP-Adapter weights.
+
+Sample directory structure:
+```bash
+ip_adapter_sd15/
+├── image_encoder.txt
+└── ip_adapter.bin
+```
+
+### Why save the weights in a .safetensors file?
+
+The weights in `ip_adapter.bin` are stored in a nested dict, which is not supported by `safetensors`. This could be solved by splitting `ip_adapter.bin` into multiple files, but for now we have decided to maintain consistency with the checkpoint structure used in the official [h94/IP-Adapter](https://huggingface.co/h94/IP-Adapter) repo.
+
+## InvokeAI-Hosted IP-Adapters
+
+TODO(ryand): Add list