docs: add roadmap and usage examples to README (#10)

## Summary
- Adds the [liblance
RFC](lance-format/lance#6035) roadmap
with implementation status checkboxes
- Adds project description, build instructions, and C/C++ usage examples

## Test plan
- [x] Documentation-only change, no code modified

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jja725

README.md

@@ -1,2 +1,118 @@
 # lance-c
-The C binding to Lance.
+
+The C/C++ binding to [Lance](https://github.com/lancedb/lance), providing native access to the Lance columnar format via a stable C ABI and header-only C++ RAII wrappers.
+
+- **C header:** [`include/lance.h`](include/lance.h)
+- **C++ wrappers:** [`include/lance.hpp`](include/lance.hpp) (header-only, RAII, exceptions)
+- **Data exchange:** [Arrow C Data Interface](https://arrow.apache.org/docs/format/CDataInterface.html) for zero-copy interop
+
+## Roadmap
+
+Based on the [liblance RFC](https://github.com/lance-format/lance/discussions/6035).
+
+### Phase 1: Core Read Path + C++ Wrappers (MVP)
+
+| Status | Component | Description |
+|--------|-----------|-------------|
+| [x] | Infrastructure | `lance-c` crate with Cargo.toml, Tokio runtime initialization |
+| [x] | Error handling | Thread-local error codes/messages for cross-FFI safety |
+| [x] | C header | `lance.h` with Arrow C Data Interface structs |
+| [x] | Dataset operations | Open/close with URI + storage options + version support |
+| [x] | Schema export | Arrow C Data Interface for zero-copy schema exchange |
+| [x] | Scanner builder | Column projection, SQL filters, limit/offset, batch size, row ID, fragment filtering |
+| [x] | ArrowArrayStream export | `lance_scanner_to_arrow_stream()` blocking API |
+| [x] | Batch iteration | `lance_scanner_next()` blocking function |
+| [x] | Poll + waker iteration | `lance_scanner_poll_next()` for async engines (Velox, Presto) |
+| [x] | Random access | Index-based row retrieval via `lance_dataset_take()` |
+| [x] | C++ wrappers | Header-only RAII library (`lance::Dataset`, `lance::Scanner`, `lance::Batch`) |
+| [x] | Builder pattern | Fluent Scanner API (`.limit().offset().batch_size().with_row_id()`) |
+
+### Phase 2: Vector Search & Indexing
+
+| Status | Component | Description |
+|--------|-----------|-------------|
+| [ ] | Vector search | Nearest-neighbor via scanner with metric/k/nprobes |
+| [ ] | Full-text search | FTS queries through scanner interface |
+| [ ] | Vector index creation | IVF_PQ, IVF_FLAT, IVF_SQ, HNSW variants |
+| [ ] | Scalar index creation | BTree, Bitmap, Inverted, Label-List indexes |
+| [ ] | Index management | List and drop index operations |
+| [ ] | C++ wrappers | `create_vector_index()` and `create_scalar_index()` methods |
+
+### Phase 3: Write Path & Mutations
+
+| Status | Component | Description |
+|--------|-----------|-------------|
+| [ ] | Dataset write | Create / append / overwrite from ArrowArrayStream |
+| [x] | Fragment writer | Batch-at-a-time fragment file writing (no commit) via `lance_write_fragments()` |
+| [ ] | Delete operations | Predicate-based deletion |
+| [ ] | Update operations | Expression-based row updates |
+| [ ] | Merge-insert | Upsert functionality with builder pattern |
+| [ ] | Schema evolution | Add/drop/alter columns with expressions |
+| [ ] | Version management | Checkout, restore, list version operations |
+
+### Phase 4: Advanced Features
+
+| Status | Component | Description |
+|--------|-----------|-------------|
+| [x] | Fragment-level access | Fragment enumeration, ID listing, scanner fragment filtering |
+| [ ] | Compaction | Fragment consolidation operations |
+| [ ] | Statistics export | Row counts, column stats for query planning |
+| [x] | Cloud storage | S3, GCS, Azure via storage options pass-through |
+| [ ] | Package distribution | vcpkg and Conan recipe packaging |
+
+### Additional (not in RFC)
+
+| Status | Component | Description |
+|--------|-----------|-------------|
+| [x] | Async scan | Callback-based `lance_scanner_scan_async()` for non-blocking scans |
+| [x] | Dataset metadata | `lance_dataset_version()`, `lance_dataset_count_rows()`, `lance_dataset_latest_version()` |
+
+## Building
+
+```bash
+cargo build --release
+```
+
+The build produces `liblance_c.{so,dylib,dll}` and the headers in `include/`.
+
+## Usage
+
+### C
+
+```c
+#include "lance.h"
+
+LanceDataset* ds = lance_dataset_open("data.lance", NULL, 0);
+if (!ds) {
+    printf("Error: %s\n", lance_last_error_message());
+    return 1;
+}
+
+struct ArrowArrayStream stream;
+LanceScanner* scanner = lance_scanner_new(ds, NULL, NULL);
+lance_scanner_to_arrow_stream(scanner, &stream);
+// consume stream...
+
+lance_scanner_close(scanner);
+lance_dataset_close(ds);
+```
+
+### C++
+
+```cpp
+#include "lance.hpp"
+
+auto ds = lance::Dataset::open("data.lance");
+printf("rows: %llu, version: %llu\n", ds.count_rows(), ds.version());
+
+ArrowArrayStream stream;
+ds.scan()
+  .limit(100)
+  .batch_size(1024)
+  .to_arrow_stream(&stream);
+// consume stream...
+```
+
+## License
+
+Apache-2.0