Add unique constraint, None option

docs/constraints.rst

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+Constraints
+================
+
+One of the most useful features provided by SQLite is the concept of
+*constraints*. Constraints signal the SQL engine that the values hold in a
+specific column **MUST** abide by specific constraints, these might be
+
+* Values of this column cannot be ``NULL``. (``NOT NULL``)
+* Values of this column cannot be repeated. (``UNIQUE``)
+* Values of this column must fulfill a condition. (``CHECK``)
+* Values of this column can be used to identify a record. (``PRIMARY``)
+* Values of this column has a default value. (``DEFAULT``)
+
+Some of these constraints are already implemented in datalite. With all of the set,
+is planned to be implemented in the future.
+
+Default Values
+---------------
+
+Columns can be given default values. This is done the same way you would give a
+datafield a default value.
+
+.. code-block:: python
+
+    @datalite("db.db")
+    @dataclass
+    class Student:
+        id_: int
+        name: str = "Albert Einstein"
+
+Therefore, from now on, any ``Student`` object, whose name is not specified, will
+have the default name ``"Albert Einstein"`` and if ``.create_entry()`` method is
+called on them, the newly inserted record will, by default, have this value in its
+corresponding column.
+
+Unique Values
+--------------
+
+Declaring a  field unique is done by a special ``TypeVar`` called ``Unique``
+this uniqueness check is done in the database level, this introduces has some pros,
+but also some cons.
+
+Pushing the uniqueness check to the database level introduces a better ability to
+handle concurrency for applications with large traffic, however, this check only
+runs when an item is registered, which means no problem will raise in
+the object creation *even if* another object of the same type with the same value
+hold in the unique field exists, no error will raise. However, if another *record*
+with the same value in the unique field is recorded in the bound database, upon
+the invocation of the ``.create_entry()`` will raise the ``ConstraintFailedError``
+exception.
+
+Uniqueness constraint is declared thusly:
+
+.. code-block:: python
+
+    @datalite("db.db")
+    @dataclass
+    class Student:
+        id_: Unique[int]
+        name: str = "Albert Einstein"
+
+Hinting a field with the ``Unique[T]`` type variable will introduce two rules:
+
+#.  The values in the column of the field in the table that represents the dataclass ``Student`` in the bound database ``db.db`` cannot be NULL, thus the corresponding field cannot be assigned ``None``.
+#.  These same values **must** be unique for each and every record.
+
+Failure of any of these two rules will result in a ``ConstraintFailedError`` exception.