Basic full text search capabilities (#62)

This is v1 of integrating full text search index into LanceDB.

# API
The query API is roughly the same as before, except if the input is text
instead of a vector we assume that its fts search.

## Example
If `table` is a LanceDB LanceTable, then:

Build index: `table.create_fts_index("text")`

Query: `df = table.search("puppy").limit(10).select(["text"]).to_df()`

# Implementation
Here we use the tantivy-py package to build the index. We then use the
row id's as the full-text-search index's doc id then we just do a Take
operation to fetch the rows.

# Limitations

1. don't support incremental row appends yet. New data won't show up in
search
2. local filesystem only 
3. requires building tantivy explicitly

---------

Co-authored-by: Chang She <chang@lancedb.com>

docs/src/fts.md

@@ -0,0 +1,50 @@
+# [EXPERIMENTAL] Full text search
+
+LanceDB now provides experimental support for full text search.
+This is currently Python only. We plan to push the integration down to Rust in the future
+to make this available for JS as well.
+
+## Installation
+
+To use full text search, you must install the fts optional dependencies:
+
+`pip install lancedb[fts]`
+
+
+## Quickstart
+
+Assume:
+1. `table` is a LanceDB Table
+2. `text` is the name of the Table column that we want to index
+
+To create the index:
+
+```python
+table.create_fts_index("text")
+```
+
+To search:
+
+```python
+df = table.search("puppy").limit(10).select(["text"]).to_df()
+```
+
+LanceDB automatically looks for an FTS index if the input is str.
+
+## Multiple text columns
+
+If you have multiple columns to index, pass them all as a list to `create_fts_index`:
+
+```python
+table.create_fts_index(["text1", "text2"])
+```
+
+Note that the search API call does not change - you can search over all indexed columns at once.
+
+## Current limitations
+
+1. Currently we do not yet support incremental writes.
+If you add data after fts index creation, it won't be reflected
+in search results until you do a full reindex.
+
+2. We currently only support local filesystem paths for the fts index.