*: add more package-specific documentation

For the most important packages at least.

pkg/encoding/address/doc.go

@@ -0,0 +1,4 @@
+/*
+Package address implements conversion of script hash to/from Neo address.
+*/
+package address

pkg/encoding/bigint/doc.go

@@ -0,0 +1,4 @@
+/*
+Package bigint implements Neo-specific big.Int (de)serialization to/from []byte.
+*/
+package bigint

pkg/encoding/fixedn/doc.go

@@ -0,0 +1,4 @@
+/*
+Package fixedn implements fixed point integers with arbitrary precision.
+*/
+package fixedn