Add target="_blank" to github link

This makes so clicking the link to the github opens in a new tab or windows.

I just keep getting frustrated clicking the link to github and then losing my place in my home timeline >_> that's all

app/assets/javascripts/components/features/getting_started/index.jsx

@@ -44,7 +44,7 @@ const GettingStarted = ({ intl, me }) => {
           <p><FormattedMessage id='getting_started.about_addressing' defaultMessage='You can follow people if you know their username and the domain they are on by entering an e-mail-esque address into the form at the top of the sidebar.' /></p>
           <p><FormattedMessage id='getting_started.about_shortcuts' defaultMessage='If the target user is on the same domain as you, just the username will work. The same rule applies to mentioning people in statuses.' /></p>
           <p><FormattedMessage id='getting_started.about_developer' defaultMessage='The developer of this project can be followed as Gargron@mastodon.social' /></p>
-          <p><FormattedMessage id='getting_started.open_source_notice' defaultMessage='Mastodon is open source software. You can contribute or report issues on github at {github}' values={{ github: <a href="https://github.com/tootsuite/mastodon">tootsuite/mastodon</a> }} /></p>
+          <p><FormattedMessage id='getting_started.open_source_notice' defaultMessage='Mastodon is open source software. You can contribute or report issues on github at {github}' values={{ github: <a href="https://github.com/tootsuite/mastodon" target="_blank">tootsuite/mastodon</a> }} /></p>
         </div>
       </div>
     </Column>