Add an Example for ReactionEngine Class

[Bin-doot]

Please add an example for ReactionEngine Class.

TODO (optional): Describe a specific scenario you would like to see addressed.

Help Topic: https://kazuyaujihara.github.io/NCDK/html/T_NCDK_Reactions_ReactionEngine.htm

[k-ujihara]

I recommend using RDKit or the newest version of CDK to handle reactions.