add install instruction

Author: mxie32

README.md

@@ -1,2 +1,17 @@
 # gtsam-project-python
 Project template using GTSAM + python wrapping
+
+
+INSTALL
+=======
+- Python3 is required
+- Make the following changes before build and install:
+    - enable `GTSAM_INSTALL_CYTHON_TOOLBOX`
+    - if you want to build the gtsam python library for a specific python version (eg 3.6), use the `-DGTSAM_PYTHON_VERSION=3.6` option, otherwise the default interpreter will be used.
+- Run `cmake ..`
+- Run `make install` (may need use sudo), and the wrapped module will be installed to `GTSAM_CYTHON_INSTALL_PATH`, which is
+by default: `<your CMAKE_INSTALL_PREFIX>/cython`
+- After build and install gtsam, navigate to `GTSAM_CYTHON_INSTALL_PATH` and run `python setup.py install`
+
+For more detailed information, please see 
+https://github.com/borglab/gtsam/tree/develop/cython