Intro fix

Hi,

I'm planning to revise some English parts of the documentation, and this is my first attempt. Here are the main things that I changed:

1- I capitalized Beego, to reflect that it's a proper noun. 
2- I rephrased some of the sentences so they flow better. 

Please let me know what you think and if you'd like me to continue this approach to revise the rest of the documentation.