NOTICE: Adding notices about third party software bundled into BuildStream

The bst2html.py tool we use for generating nice documentation is a work
derived from an upstream work licensed under MIT.

The completions functionality in the click library we depend on is
insufficient and as such we've forked our own implementation, this is
thus a derived work of click which is licensed under BSD.
diff --git a/NOTICE b/NOTICE
index 8266ca0..f104d0c 100644
--- a/NOTICE
+++ b/NOTICE
@@ -3,3 +3,17 @@
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).
+
+
+Software deriving from third parties:
+
+  * doc/bst2html.py
+
+    This file is largely derived from https://github.com/Kronuz/ansi2html.git
+    which is under the MIT license and as such is acceptable to redistribute
+    under the Apache License.
+
+  * src/buildstream/_frontend/complete.py
+
+    This file is forked from the click library which is under BSD license
+    and as such is acceptable to redistribute under the Apache License.