Add docs for types

docs/types.rst

@@ -0,0 +1,53 @@
+======================
+Decsion Variable Types
+======================
+
+Real Valued
+-----------
+
+Real valued variables are expressed as floating-point numbers between
+some minimum and maximum bounds.  For example, in the following, we
+configure the bounds as `[-10, 10]`:
+
+.. literalinclude:: ../examples/custom_problem_function.py
+   :language: python
+
+Binary
+------
+
+Binary variables store a "binary string".  Traditionally, this is
+represented as `0`s and `1`s, but in Platypus, it is stored as an
+array of `False` and `True` values.  The knapsack problem demonstrates
+the binary representation, where an item is selected when the binary
+value is `True`.  The goal, then, is to maximize the profit of the
+selected items while being constrained by the total capacity.
+
+.. literalinclude:: ../examples/knapsack.py
+   :language: python
+
+Integer
+-------
+
+The integer variable is a mix between `Real` and `Binary`.  The variable
+is constructed with minimum and maximum bounds, but is internally represented
+as a Gray-coded binary variable.  The Gray-coding ensures each adjacent
+integer value can be produced by a single bit mutation.
+
+Permutation
+-----------
+
+The permutation variable creates a permutation of a list of items.  That is,
+each item must appear in the list, but the ordering can change.  Here we
+construct a permutaton of the numbers `0` through `10` using the `range`
+functon.  However, note that the items is not limited to a list of integers,
+any collection of objects can be provided.
+
+.. literalinclude:: ../examples/permutaton.py
+   :language: python
+
+Subset
+------
+
+Lastly we have the subset.  Similar to the mathematical `choose(n, k)` operation,
+a subset variable represents a fixed-size selection of size `k` from a
+collection of `n` items.