Refine Contributing Guide page

[naveenchkrvrthy]

As a new contributor, the Contributing Guide page was the first page I looked at and I noticed a few inconsistencies while reading through it:

• Page title can be changed to Contributor guide. Contributing is a verb, and it would be better to use a noun (contributor) here to describe the guide.
• Periods are missing for some bullet points
• Getting Started bullet list can be a numbered list
• Headings can be rewritten as an 'action' heading for better SEO

[abrahamalen]

like to work on this issue @naveenchkrvrthy