Complete rework of README.md

[jay-em-Ach]

• Converted full file to markdown
• Added instructions for all of the steps required to use the integration in HA. I did this make it more clear for others because I had trouble figuring it out on my own.
• Should resolve Issue #3

[dylandoamaral]

Thank you, it clearly explains how the integration works.

I just have three comments before approving it:

• The "requirements" are right since there is today only one category of sensor (upcomming) corresponding to the upcomming media card. However I am planning to create more kind of sensors depending of the users needs. (Just to make things clear)
• Converting the full file to markdown is a good idea, however, I did this for a particular reason. I want the header to be horizontally centered and we can't do that using only markdown.
• Why did you remove the emojis ?

The last two points are important to me because I want to keep consistency with my other home assistant libraries (https://github.com/dylandoamaral/uptime-card). If this is a problem then we will have to make a new PR for each repositories.

[jay-em-Ach]

@dylandoamaral, feel free to edit my PR as you see fit.

[dylandoamaral]

That's perfect thank you very much 😀

Close #3