feat(python): add exist_ok option to create table (#813)

This mimics CREATE TABLE IF NOT EXISTS behavior.
We add `db.create_table(..., exist_ok=True)` parameter.
By default it is set to False, so trying to create
a table with the same name will raise an exception.
If set to True, then it only opens the table if it
already exists. If you pass in a schema, it will
be checked against the existing table to make sure
you get what you want. If you pass in data, it will
NOT be added to the existing table.

docs/src/guides/tables.md

@@ -31,13 +31,23 @@ This guide will show how to create tables, insert data into them, and update the
     ```
 
     !!! info "Note"
-        If the table already exists, LanceDB will raise an error by default. If you want to overwrite the table, you can pass in mode="overwrite" to the createTable function.
+        If the table already exists, LanceDB will raise an error by default. 
+
+        `create_table` supports an optional `exist_ok` parameter. When set to True
+        and the table exists, then it simply opens the existing table. The data you
+        passed in will NOT be appended to the table in that case.
+
+        ```python
+        db.create_table("name", data, exist_ok=True)
+        ```
+        
+        Sometimes you want to make sure that you start fresh. If you want to 
+        overwrite the table, you can pass in mode="overwrite" to the createTable function.
 
         ```python
         db.create_table("name", data, mode="overwrite")
         ```
 
-
     ### From pandas DataFrame
 
     ```python