Add Tooltip template

[navneetrai]

• Adds a template for Tooltips
• Replace placeholders in templates
• Updates Readme

Note: The following issues should still be addressed to show Additional data not available in Charts in tooltips

• How can we pass addon data for each tooltip point (i.e. fir each model, we need to pass add-on data for each data point)
• Should we make this addon data editable.
• How can we use this data in tooltip. I believe we can achieve this by adding another wildcard placeholder in tooltip to replace any additional data

I tried a few different approaches to pass additional data, but this seems to be a discussion based on requirements. I believe we should either:-

1. Assign a JSON field from which we can pull addon data.
2. Add a new field in our Chart model to store addon data.
3. Add a method to pull addon data for each datapoint and leave it's implementation on user.