fix: add LICENSE (#133)

astro-portabletext/LICENSE

@@ -0,0 +1,15 @@
+ISC License (ISC)
+
+Copyright 2022 - CURRENT Tom Theisel <tom.theisel@gmail.com>
+
+Permission to use, copy, modify, and/or 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.