added limitations section

docs/index.rst

@@ -14,6 +14,7 @@ Contents:
    advanced
    cmake
    benchmark
+   limitations
    faq
    reference
    changelog

docs/limitations.rst

+Limitations
+###########
+
+pybind11 strives to be a general solution to binding generation, but it also has
+certain limitations:
+
+- pybind11 casts away ``const``-ness in function arguments and return values.
+  This is in line with the Python language, which has no concept of ``const``
+  values. This means that some additional care is needed to avoid bugs that
+  would be caught by the type checker in a traditional C++ program.
+
+- Multiple inheritance relationships on the C++ side cannot be mapped to
+  Python.
+
+Both of these features could be implemented but would lead to a significant
+increase in complexity. I've decided to draw the line here to keep this project
+simple and compact. Users who absolutely require these features are encouraged
+to fork pybind11.
+