Show default values for optional parameters if possible

The reflection API lets us check to see if a parameter has a default
value, or if it accepts null if it’s an optional parameter.  So we
check for those two cases and add the default value or null if either
situation is true.  That said, this code seems to have no effect.
Testing it out on various standard library functions with defined
default values does not produce those default values in the snippets,
i.e. isDefaultValueAvailable() returns false even though the official
documentation gives a default value.
Make a small readability improvement in the README

Adding ‘then’ to that sentence makes it easier to read since two
chunks of example code interrupt it.
Add an example with optional parameters to the README
Place the ‘$’ sigil inside the square braces for optional parameters

This makes the display of optional parameters more in sync with the
way they appear in the PHP manual, where braces enclose the entire
parameter name including the ‘$’ sigil.
Bump the pre-release version number for optional parameter support