Added a link to resource.openra.net

as described in #108 it was not directly linked from the main website. The bug reporting was redundant to the help section and the tell your friends was redundant to the get in touch via social networks.
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<p>You can share custom content such as maps on the <a href="http://resource.openra.net/">OpenRA Resource Center</a>.<br />Maps uploaded to this site are automatically shared to our dedicated servers, and can be downloaded on-demand when connecting to a server.</p>
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