Add license and badges

LICENSE

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+pg_wait_sampling is released under the PostgreSQL License, a liberal Open Source license, similar to the BSD or MIT licenses.
+
+Copyright (c) 2015-2017, Postgres Professional
+Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
+Portions Copyright (c) 1994, The Regents of the University of California
+
+Permission to use, copy, modify, and distribute this software and its documentation for any purpose, without fee, and without a written agreement is hereby granted, provided that the above copyright notice and this paragraph and the following two paragraphs appear in all copies.
+
+IN NO EVENT SHALL POSTGRES PROFESSIONAL BE LIABLE TO ANY PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN IF POSTGRES PROFESSIONAL HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+POSTGRES PROFESSIONAL SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND POSTGRES PROFESSIONAL HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.