Update welcome.md

- The Helpful Posts from Steemit Users section has been entirely removed.  The majority of the posts had a very large number of comments which rendered them useless when loading on mobile devices.  Others were subjective or quite outdated. 

- Replaced slang terms like 'hang out' to allow for a clearer translation into other languages.

- The https://steemit.com/trending30/introduceyourself URL is incorrect.  Fixed. 

- Grammatical and spelling errors fixed.

- https://www.steemithelp.net/ link removed. The site is out of date (ie. informs users they can edit a post for 30 days). 

- Steem markdown course link added.