Update CONTRIBUTING.md

CONTRIBUTING.md

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 # Contribution Guidelines
 Unfortunately, not every library, tool or framework can be considered for inclusion. The aim of Awesome PHP is to be a concise list of noteworthy and interesting software written in modern PHP. Therefore, suggested software should: 
 
-a) Be widely recommended regardless of personal opinion
-b) Well known or discussed within the PHP community
-c) Be unique in its approach or function
-d) Fill a niche gap in the market  
+1. Be widely recommended regardless of personal opinion
+2. Well known or discussed within the PHP community
+3. Be unique in its approach or function
+4. Fill a niche gap in the market  
 
 Self-promotion is frowned upon, so please consider seriously whether your project meets the criteria before opening a pull request, otherwise it may be closed without being reviewed.