docs(readme): document OpenAI-compatible endpoint configuration (#168)

Add an "OpenAI-compatible (custom endpoint)" section under Embedding
Models showing the `openai/` prefix plus `OPENAI_BASE_URL` /
`OPENAI_API_KEY` env vars, and link the LiteLLM "Setting API Keys" doc
from the general LiteLLM note so users of any provider can find the
canonical env-var list.

Author: georgeh0

README.md

@@ -524,6 +524,23 @@ envs:
 
 </details>
 
+<details>
+<summary>OpenAI-compatible (custom endpoint)</summary>
+
+Many providers (vLLM, LM Studio, LocalAI, Together, Fireworks, DeepInfra, …) expose an OpenAI-compatible embedding API. Use the `openai/` prefix and point `OPENAI_BASE_URL` at your endpoint:
+
+```yaml
+embedding:
+  model: openai/your-model-name
+envs:
+  OPENAI_BASE_URL: https://your-endpoint/v1
+  OPENAI_API_KEY: your-api-key
+```
+
+Don't append `/embeddings` to the base URL — LiteLLM handles that.
+
+</details>
+
 <details>
 <summary>Azure OpenAI</summary>
 
@@ -612,7 +629,7 @@ envs:
 
 </details>
 
-Any [LiteLLM-supported model](https://docs.litellm.ai/docs/embedding/supported_embedding) works. When using a LiteLLM model, set `provider: litellm` (or omit `provider` — LiteLLM is the default for non-`sentence-transformers` models).
+Any [LiteLLM-supported model](https://docs.litellm.ai/docs/embedding/supported_embedding) works. When using a LiteLLM model, set `provider: litellm` (or omit `provider` — LiteLLM is the default for non-`sentence-transformers` models). For the full list of env vars each provider reads (API keys, base URLs, regions, …), see LiteLLM's [Setting API Keys](https://docs.litellm.ai/docs/set_keys).
 
 ### Local SentenceTransformers Models