README.md: Mention Python.h is needed

README.md

@@ -19,9 +19,10 @@
 * cmake (needed for bundled capnproto)
   - ninja (macOS + Linux)
   - Visual Studio 2017+
-
 * capnproto-0.8.0 (0.7.0 will also work if linking to system libraries)
   - Not necessary if using bundled capnproto
+* Python development headers (i.e. Python.h)
+  - Distributables from python.org include these, however they are usually in a separate package on Linux distributions
 
 32-bit Linux requires that capnproto be compiled with `-fPIC`. This is usually set correctly unless you are compiling canproto yourself. This is also called `-DCMAKE_POSITION_INDEPENDENT_CODE=1` for cmake.