clarify that tor's license is free software / open source

One of our Tor people was just asking whether Tor's license is OSI-approved.

The answer is yes, it's distributed under the 3-clause bsd license: https://gitweb.torproject.org/tor.git/tree/LICENSE#n12

But that description in the license file could be even clearer, for people who don't already recognize the text of the 3-clause bsd license.

I propose this change:

diff --git a/LICENSE b/LICENSE
index cc27668427..438208426c 100644
--- a/LICENSE
+++ b/LICENSE
@@ -9,7 +9,8 @@
         there may be other license terms that you should be aware of.
 
 ===============================================================================
-Tor is distributed under this license:
+Tor is distributed under the "3-clause BSD" license, a commonly used
+software license that means Tor is both free software and open source:
 
 Copyright (c) 2001-2004, Roger Dingledine
 Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson

I admit there is a bit of politics in my proposed change, but it's hard to describe free software licenses without a bit of politics creeping in, and this seems to me like a balanced approach.

Nick, feel free to do the commit if you're ok with it, or tell me and I will.