Add installation information to the README

README.md

@@ -5,6 +5,8 @@
 Numba special allows SciPy's special functions to be used in
 Numba.
 
+## How to use it
+
 ```python
 >>> import numba
 >>> import scipy.special as sc
@@ -20,3 +22,11 @@ Numba.
 For detailed information on which functions can be used from Numba,
 check the
 [documentation](https://numba-special.readthedocs.io/en/latest/).
+
+## Installing
+
+Building and installing `numba_special` requires Cython, SciPy, and
+Numba. Until the fix for [this
+issue](https://github.com/numba/numba/issues/4133) is included in a
+Numba release, `numba_special` must be built against the master branch
+of Numba.