Update README to include Colab links

Add links to Colab notebooks that are replicates of some of the notebooks in /ipython.

README.md

@@ -13,6 +13,12 @@ conda install -c xiaoruidong rdmc
 
 For detailed APIs, please check the [documentation](https://rdmc.readthedocs.io).
 
+## Demos
+Feel free to check demos in the `ipython/`, some of them are also available on the Google Colab:
+- [Generate Atom Map for Reactions](https://colab.research.google.com/drive/19opX3Sr4R24o9n8f1o4LMSqlVIwN83xk?usp=sharing)
+- [Handle molecule from/to XYZ](https://colab.research.google.com/drive/1QbmdvUMQqByPBDQVW7xTlp2rXg9EJ2_J?usp=sharing)
+- [Parse QM Results](https://colab.research.google.com/drive/1JnTzETOGE3R3Q_foOLsnFgeN883J36dl?usp=sharing)
+
 ## Requirements
 * python
 * numpy
@@ -24,6 +30,3 @@ For detailed APIs, please check the [documentation](https://rdmc.readthedocs.io)
 * networkx
 * matplotlib
 * cclib
-
-## Demos
-Feel free to check demos in the `ipython/` to learn some basic use of RDMC repository