fix: doc: API cli change not documented about implicit HEAD remov…

…ed in revision list specifier. (fixes vaab#81)

In 2.5.1, ``gitchangelog show ^3.0.0`` command would implicitly add a
``HEAD`` in the revlist specifiers, effectively being equivalent to
``0.0.3..HEAD``.

This behavior is removed in 3.0.0+ to stick to ``git rev-list REVLIST``
syntax.  As a consequence, ``gitchangelog ^3.0.0`` won't select any
revision and thus will cast an error about no commits matching revlist.

README.rst

@@ -439,7 +439,7 @@ REVLIST``. Examples follows::
     gitchangelog 0.0.2..0.0.3
 
     ## will output only tags since 0.0.3 (excluded)
-    gitchangelog ^0.0.3
+    gitchangelog ^0.0.3 HEAD
 
     ## will output all tags up to 0.0.3 (included)
     gitchangelog 0.0.3