Add post message example

[hodovani]

An example of html web page that get a post message and show it in alert. https://github.com/hodovani/iframe-integration-example/blob/main/index.html

To run this example please:

• Clone this repo https://github.com/hodovani/iframe-integration-example.
• Run python3 -m http.server in the same folder.
• Run optimade.science locally.
• Visit http://localhost:5000/?filter=nelements%3D2&providers=threedd#threedd-144.
• Enter an URL localhost:8000 in the URL field.
• You should get an alert with a message "message".

https://user-images.githubusercontent.com/12833067/106393157-2a73b080-63fe-11eb-8fe3-296600e5afdc.mov

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[blokhin]

@hodovani much appreciated :+1: