added documentation for examples in dp

[dnabanita7]

Description

Added required documentation i.e. description of demos and examples in README.md. The issue linked is #312 .

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [ ] I have labeled this PR with the relevant Type labels
• [x] My changes are covered by tests

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