Write section in the top of README.md about difference between tenv and asdf

[kvendingoldo]

[dvaumoron]

@kvendingoldo, what do you think about my changes ? should i add more details ?

[kvendingoldo]

I like them. I think that we need to add a little bit more details, because ppl ask me about the differences when they starting to use tenv. For the beginning it's enough, thank you so much. I will think about it a little bit more this evening and close this issue.