Updated contribution guidelines with note about PR titles

CONTRIBUTING.md

 
 * Please search previous suggestions before making a new one, as yours may be a duplicate.
 * Libraries that are PHP 5.3+, Composer-installable, PSR compliant, tested and documented are preferred.
+* Enter a meaningful pull request description.
 * Please make an individual commit for each suggestion in a pull request.
-* Put a link to each library in your pull request ticket, so they're easier to look at.
+* Put a link to each library in your pull request ticket so they're easier to look at.
 * Use the following format for libraries: \[LIBRARY\]\(LINK\) - DESCRIPTION.
 * Prefix duplicate library names with their vendor or namespace followed by a space: Foo\Bar would be Foo Bar.
-* New categories, or improvements to the existing categorisation are welcome.
+* New categories, or improvements to the existing categorisation, are always welcome.
 * Keep descriptions short and simple. 
 * End all descriptions with a full stop/period.
 * Check your spelling and grammar.