Add note for compiling eigenpy with python3 on macOS

Getting eigenpy (and pinocchio) to compile on macOS with python3 took me some effort. The required trick turned out to "only" add `-DPYTHON_EXECUTABLE=\`which python3\`` to the cmake argument.

Do you think this is worth adding to the README.md/installation instructions? I am happy to provide something similar for pinocchio as well.

Author: jviereck

README.md

@@ -23,6 +23,23 @@ directory:
 Please note that CMake produces a `CMakeCache.txt` file which should
 be deleted to reconfigure a package from scratch.
 
+#### Compiling for python3 on macOS
+
+Make sure you have boost for python3 installed. If you use homebrew, you can install it via `brew install boost-python3`.
+
+Assuming you have python2 and python3 installed on your system, you can compile for python3 using
+
+```
+cmake -DPYTHON_EXECUTABLE=`which python3`  ..
+```
+
+In case you get an error as only the libraries for python2 are found, make sure macOS can find the python3 libraries. One way to ensure this is by adding a symbolic link to the python3 libraries like
+
+```
+ln -s /usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/libpython3.7.dylib /usr/loca/lib/libpython3.7.dylib
+```
+
+where the abolve file source path was determined by looking at the output of `brew ls --verbose python3 | grep libpython3`.
 
 ### Dependencies