Create an Experiment Developer Guidelines Document

[priya100raman]

Create an Experiment Developer Guidelines Document and publish it as README in the experiment repository in the
ph3-exp-template repository.

Include the following -

1. Link to Best practices
2. Debugging build failure.
3. Populating pretest and posttest JSON files.
4. Using Validation Tool
5. Using Performance Tool

[bhavberi]

It would be great if more information about the virtual labs common style sheet & js assets, about the various parameters in the experiment-descriptor file can also be added as Experiment Developer Guidelines Document.