User's guide

[Leamsi9]

Add a brief intro that tells users how they might use the extensions to green their digital footprint and become responsible website consumers. To include:

• briefly touch on how websites create emissions and how impactful this is, with a link for more info.
• the concept of Green Mode Design and how extensions can empower users to control their digital CO2 footprint
• the main functionalities this repo covers (CO2 measurement, energy optimisation, graceful degradation and carbon offsetting)
• practical example of how to implement the functions above by combining specific extensions.
• Disclaimer on security and privacy and that we list what is publicly available but do not vet each app and users should use their own discretion.
• CTA to contribute more extensions.