Add API documentation for graph retrieval, query, and commit history endpoints

Migrated from FalkorDB/code-graph-backend PR #96.
Original issue: FalkorDB/code-graph-backend#83
Resolves #534

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gkorland

README.md

@@ -246,4 +246,41 @@ A C analyzer exists in the source tree, but it is commented out and is not curre
 
 This project is licensed under the MIT License - see the [LICENSE](LICENSE) file for details.
 
+Knowledge Graph, Code Analysis, Code Visualization, Dead Code Analysis, Graph Database
+
+## API Reference
+
+### Retrieve a graph
+
+Fetch graph entities from a repository:
+
+```bash
+curl -X GET "http://127.0.0.1:5000/graph_entities?repo=<REPO_NAME>" -H "Authorization: ${SECRET_TOKEN}"
+```
+
+Example:
+```bash
+curl -X GET "http://127.0.0.1:5000/graph_entities?repo=GraphRAG-SDK" -H "Authorization: ${SECRET_TOKEN}"
+```
+
+### Send Query
+
+Query your code graph using natural language:
+
+```bash
+curl -X POST http://127.0.0.1:5000/chat -H "Content-Type: application/json" -d '{"repo": "<REPO_NAME>", "msg": "<YOUR_QUESTION>"}' -H "Authorization: ${SECRET_TOKEN}"
+```
+
+### History change
+
+List all commits:
+```bash
+curl -X POST http://127.0.0.1:5000/list_commits -H "Content-Type: application/json" -d '{"repo": "<REPO_NAME>"}' -H "Authorization: ${SECRET_TOKEN}"
+```
+
+Switch to a specific commit:
+```bash
+curl -X POST http://127.0.0.1:5000/switch_commit -H "Content-Type: application/json" -d '{"repo": "<REPO_NAME>", "commit": "<COMMIT_HASH>"}' -H "Authorization: ${SECRET_TOKEN}"
+```
+
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