Improve grammar on README/website

Well, I'm not a native English speaker, so readme and website may contain some spelling mistakes or sentences that could be written in a better way.

Just as I always try to write correctly in my native language (Portuguese), I do with English. But, well, until I master the language, I will need some help. 😅

I will appreciate very much people doing some review. Also, this is another way for beginners to contribute.

I'm leaving this issue here *ad eternum* so we can constantly improve that.