Add note to docs explaining that this can be used in a schema-less manner similar to Accumulo

Each Sleeper table has a fixed schema. Superficially this appears to mean that it cannot be used in the schema-less way that Accumulo can be used. However that is not true. Accumulo has a fixed schema (row-key is a byte array, column family is a byte array, etc), but this can be used to create the illusion of a schema-less system by putting a different cell in each line (e.g. row-key = abc, column = age, value = 21; row-key = abc, column = height, value = 150; row-key = xyz, column = address, value = 1 High Street). In this way Accumulo can be used to store arbitrary "columns" which are not specified at table creation time. However, any fixed schema can be used in a similar way. Specifically, using a Sleeper schema of row-key is a byte array, column family is a byte array, etc, allows data to be stored in a similar way to Accumulo, i.e. without requiring specifying the columns at table creation time.