table.count property, plus made a start on table documentation

tests/test_introspect.py

@@ -0,0 +1,28 @@
+from sqlite_utils import db
+import sqlite3
+import pytest
+
+
+@pytest.fixture
+def existing_db():
+    database = db.Database(sqlite3.connect(":memory:"))
+    database.conn.executescript(
+        """
+        CREATE TABLE foo (text TEXT);
+        INSERT INTO foo (text) values ("one");
+        INSERT INTO foo (text) values ("two");
+        INSERT INTO foo (text) values ("three");
+    """
+    )
+    return database
+
+
+def test_count(existing_db):
+    assert 3 == existing_db["foo"].count
+
+
+def test_columns(existing_db):
+    table = existing_db["foo"]
+    assert [{"name": "text", "type": "TEXT"}] == [
+        {"name": col.name, "type": col.type} for col in table.columns
+    ]