README updates

[avinash-vk]

There are a ton of commands that the bot uses. Would be helpful to have execution samples and command description/workflows in the README.

[Jittojoyes98]

The use of ;start-session will be great if we provide with question tag as well.

[avinash-vk]

Could you please elaborate on what exactly you mean? @Jittojoyes98

[Jittojoyes98]

We can choose questions based on difficulty as well as based on topics.Like dynamic programming,graph theory etc. Leetcode too support this,you can checkout their site.So we can practice problems based on difficulty as well as problem tags.It will be better if we use a different command for this.

[avinash-vk]

Thanks @Jittojoyes98
Right! I agree on that, it will definitely be better and a really good feature to have.
You can select questions based on difficulty, right now for example, ;challenge-me 2 would return a question with a difficulty of medium from leetcode.
But will definitely look into the topics part of it! Please feel free to suggest anything more :)
Thanks for contributing 🚀

Opened #3 for the same