Update the license text on the release page to telegraph to users about the relicening.

[chandlerc]

Summary

This mostly makes it clear that there will be a change with the next release,
and that the text here only applies to the releases.

We need to remember to update this for the next release.

[hans]

lgtm

[hans]

With llvm 9 coming up, we should update this, but what should it say exactly?

index.html

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 <div class="rel_section">License</div>
 
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-<p><a href="//llvm.org">LLVM</a> is distributed under the <a
-href="7.0.0/LICENSE.TXT">University of Illinois Open Source License</a>, an <a
-href="http://www.opensource.org/licenses/UoI-NCSA.php">OSI-approved</a>
+<p>Currently, <a href="//llvm.org">LLVM</a> releases are distributed under the
+<a href="7.0.0/LICENSE.TXT">University of Illinois Open Source License</a>, an
+<a href="http://www.opensource.org/licenses/UoI-NCSA.php">OSI-approved</a>
 license.</p>
 
+<p>However, LLVM version 8 will be the last release under that license as LLVM
+is moving to a new license structure. You can find more details about this in
+the <a href="//llvm.org/docs/DeveloperPolicy.html#relicensing">developer's
+policy</a>.</p>
+
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