[openstreetmap-website] Add welcome page (#338)

[RobJN]

To add to my previous comment, the merge of the "Get a free account" and "Start Contributing" sections can be done as such (all text as per original text except where I have indicated the insertion of new words in curly brackets):

* To edit OpenStreetMap, you'll need a {free} user account. Sign up here.
* 
* With your user account, you can edit the map and collaborate with other mappers. {Simply} zoom in anywhere in the world and click the Edit button to start contributing. 

Best wishes,
Rob

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https://github.com/openstreetmap/openstreetmap-website/pull/338#issuecomment-21316940
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