Ask user for URL on registration and create feed from the URL on his/her profile page I know i should be using the Feeds module for this but i want to know how will i turn that particular URL into feed automatically when user enters his feed URL. 

Basically i want to let users show their github activity feed on their drupal profiles.
URL for it is something like this https://github.com/githubusername.atom
So in a feed, the URL still remains standard and if i just ask them to enter their github username and generate the feed accordingly, that would be really amazing. I know how to do it in php but i don't know drupal moduling. It would be great if anyone could help me with it or give me ideas on how to do it.
Since you already know how to do it in PHP, you could use the Panels module. You can then use a Panel to display the user's profile, and you can add a "Custom" field with your php inside.

You could also do something similar by using the Display Suite module and adding a custom code field.

Of course that would require you put php directly into the UI, which isn't always considered the best approach but can be done. To get the user's username, you'd put a field in their profile asking them the user name. That value would then be stored in the $account object.