add package level documentation

Author: elrayle

spdxexp/doc.go

@@ -0,0 +1,7 @@
+/*
+Spdxexp package validates licenses and determines if a license expression is satisfied by a list of licenses.
+Validity of a license is determined by the [SPDX license list].
+
+[SPDX license list]: https://spdx.org/licenses/
+*/
+package spdxexp