narrow: move changegroup.supportedoutgoingversions() override to core
Also document why we need cg3 or higher.
Differential Revision: https://phab.mercurial-scm.org/D2488
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<title>Read Me - Important Information</title>
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<p class="p1"><b>Before you install</b></p>
<p class="p2"><br></p>
<p class="p3">This is an OS X version of Mercurial that depends on the default Python installation.</p>
<p class="p2"><br></p>
<p class="p1"><b>After you install</b></p>
<p class="p2"><br></p>
<p class="p3">This package installs the <span class="s2">hg</span> executable as <span class="s2">/usr/local/bin/hg</span>. See <span class="s2">hg debuginstall</span> for more info on file locations.</p>
<p class="p2"><br></p>
<p class="p1"><b>Documentation</b></p>
<p class="p2"><br></p>
<p class="p3">Visit the <a href="https://mercurial-scm.org/">Mercurial web site and wiki</a></p>
<p class="p2"><br></p>
<p class="p3">There's also a free book, <a href="https://book.mercurial-scm.org/">Distributed revision control with Mercurial</a></p>
<p class="p2"><br></p>
<p class="p1"><b>Reporting problems</b></p>
<p class="p2"><br></p>
<p class="p3">If you run into any problems, please file a bug online:</p>
<p class="p3"><a href="https://bz.mercurial-scm.org/">https://bz.mercurial-scm.org/</a></p>
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