Added question on B.C. years

website/templates/_help.html

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 <p>Earlier versions of TimelineJS required you to enter a date as January 1 of a given year to do this, but now you should just enter the full year, as explained above.</p></div>
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+            <div class="accordion-heading"><a data-toggle="collapse" data-parent="#accordion-faq" href="#faq-18" class="accordion-toggle collapsed">How do I add B.C. dates?</a></div>
+            <div id="faq-18" style="height: 0px;" class="accordion-body collapse">
+              <div class="accordion-inner">You can add B.C. dates by using negative years. For example, -100 would be 100 B.C. Be sure to change the formatting of the cell you want to modify to Plain Text — "Format > Number > Plain Text".</div>
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             <div class="accordion-heading"><a data-toggle="collapse" data-parent="#accordion-faq" href="#faq-18" class="accordion-toggle collapsed">Can I use an Excel spreadsheet instead of Google Spreadsheets?</a></div>
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               <div class="accordion-inner">No, but it&#x27;s pretty easy to copy rows into a Google Spreadsheet. TimelineJS uses Google&#x27;s API to access the data from the Google Spreadsheet and turns it into native TimelineJS json.</div>