Update LICENSE file:

The existing license file contains copyright statements and license
snippets referencing code that has long since been removed from the
codebase and which are not necessary any longer.

The general copyright statement for the ISC License is also changed
from "Ripple Labs" which is one contributor to the codebase, to the
more general "XRP Ledger Developers".

Remaining code which was originally taken from Bitcoin includes the
relevant copyright statement(s) inline.

To aid in the automated detection of the license type by GitHub and
tools like `licensee`, the following statement which referenced the
ISC license, and which was listed at the bottom of the file, is now
incorporated at the top:

>The accompanying files incorporate work covered by the following copyright
>and previous license notice:
>
>Copyright (c) 2011 Arthur Britto, David Schwartz, Jed McCaleb,
>Vinnie Falco, Bob Way, Eric Lombrozo, Nikolaos D. Bougalis, Howard Hinnant

LICENSE.md

@@ -0,0 +1,17 @@
+ISC License 
+
+Copyright (c) 2011, Arthur Britto, David Schwartz, Jed McCaleb, Vinnie Falco, Bob Way, Eric Lombrozo, Nikolaos D. Bougalis, Howard Hinnant.
+Copyright (c) 2012-2020, the XRP Ledger developers.
+
+Permission to use, copy, modify, and distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+