Add copy of Vim license text for package distribution

Closes #172

LICENSE

@@ -0,0 +1,89 @@
+ack.vim is distributed under the same license terms as Vim itself, which you
+can find in full with `:help license` within Vim, or copied in full herein.
+
+Copyright (c) 2007-2015 Antoine Imbert <[email protected]>
+                        and contributors.
+
+Maintainers may be contacted via GitHub Issues at:
+
+   https://github.com/mileszs/ack.vim/issues
+
+
+VIM LICENSE
+
+I)  There are no restrictions on distributing unmodified copies of Vim except
+    that they must include this license text.  You can also distribute
+    unmodified parts of Vim, likewise unrestricted except that they must
+    include this license text.  You are also allowed to include executables
+    that you made from the unmodified Vim sources, plus your own usage
+    examples and Vim scripts.
+
+II) It is allowed to distribute a modified (or extended) version of Vim,
+    including executables and/or source code, when the following four
+    conditions are met:
+    1) This license text must be included unmodified.
+    2) The modified Vim must be distributed in one of the following five ways:
+       a) If you make changes to Vim yourself, you must clearly describe in
+	  the distribution how to contact you.  When the maintainer asks you
+	  (in any way) for a copy of the modified Vim you distributed, you
+	  must make your changes, including source code, available to the
+	  maintainer without fee.  The maintainer reserves the right to
+	  include your changes in the official version of Vim.  What the
+	  maintainer will do with your changes and under what license they
+	  will be distributed is negotiable.  If there has been no negotiation
+	  then this license, or a later version, also applies to your changes.
+	  The current maintainer is Bram Moolenaar <[email protected]>.  If this
+	  changes it will be announced in appropriate places (most likely
+	  vim.sf.net, www.vim.org and/or comp.editors).  When it is completely
+	  impossible to contact the maintainer, the obligation to send him
+	  your changes ceases.  Once the maintainer has confirmed that he has
+	  received your changes they will not have to be sent again.
+       b) If you have received a modified Vim that was distributed as
+	  mentioned under a) you are allowed to further distribute it
+	  unmodified, as mentioned at I).  If you make additional changes the
+	  text under a) applies to those changes.
+       c) Provide all the changes, including source code, with every copy of
+	  the modified Vim you distribute.  This may be done in the form of a
+	  context diff.  You can choose what license to use for new code you
+	  add.  The changes and their license must not restrict others from
+	  making their own changes to the official version of Vim.
+       d) When you have a modified Vim which includes changes as mentioned
+	  under c), you can distribute it without the source code for the
+	  changes if the following three conditions are met:
+	  - The license that applies to the changes permits you to distribute
+	    the changes to the Vim maintainer without fee or restriction, and
+	    permits the Vim maintainer to include the changes in the official
+	    version of Vim without fee or restriction.
+	  - You keep the changes for at least three years after last
+	    distributing the corresponding modified Vim.  When the maintainer
+	    or someone who you distributed the modified Vim to asks you (in
+	    any way) for the changes within this period, you must make them
+	    available to him.
+	  - You clearly describe in the distribution how to contact you.  This
+	    contact information must remain valid for at least three years
+	    after last distributing the corresponding modified Vim, or as long
+	    as possible.
+       e) When the GNU General Public License (GPL) applies to the changes,
+	  you can distribute the modified Vim under the GNU GPL version 2 or
+	  any later version.
+    3) A message must be added, at least in the output of the ":version"
+       command and in the intro screen, such that the user of the modified Vim
+       is able to see that it was modified.  When distributing as mentioned
+       under 2)e) adding the message is only required for as far as this does
+       not conflict with the license used for the changes.
+    4) The contact information as required under 2)a) and 2)d) must not be
+       removed or changed, except that the person himself can make
+       corrections.
+
+III) If you distribute a modified version of Vim, you are encouraged to use
+     the Vim license for your changes and make them available to the
+     maintainer, including the source code.  The preferred way to do this is
+     by e-mail or by uploading the files to a server and e-mailing the URL.
+     If the number of changes is small (e.g., a modified Makefile) e-mailing a
+     context diff will do.  The e-mail address to be used is
+     <[email protected]>
+
+IV)  It is not allowed to remove this license from the distribution of the Vim
+     sources, parts of it or from a modified version.  You may use this
+     license for previous Vim releases instead of the license that they came
+     with, at your option.