interop: add some top-level doc.go

pkg/interop/doc.go

@@ -0,0 +1,14 @@
+/*
+Package interop contains smart contract API functions.
+Its subpackages can be imported into smart contracts written in Go to provide
+various functionality. Upon compilation, functions from these packages will
+be substituted with appropriate NeoVM system calls implemented by Neo. Usually
+these system calls have additional price in NeoVM, so they're explicitly written
+in the documentation of respective functions.
+
+Note that unless written otherwise structures defined in this packages can't be
+correctly created by new() or composite literals, they should be received from
+some interop functions (and then used as parameters for some other interop
+functions).
+*/
+package interop