Add documentation for essentials

essentials/conns.go

 package essentials
 
-import "net"
+import (
+	"io"
+	"net"
+)
 
+// CloseableReader is a reader interface that can close its reading end.
 type CloseableReader interface {
+	io.Reader
+
 	CloseRead() error
 }
 
+// CloseableWriter is a writer that can close its writing end.
 type CloseableWriter interface {
+	io.Writer
+
 	CloseWrite() error
 }
 
+// Conn is an extension of net.Conn that can close its ends. This mostly
+// implies TCP connections.
 type Conn interface {
 	net.Conn
 	CloseableReader

essentials/doc.go

+// This is a minimal package that contains _essentials_ of mtglib and its
+// complimentary packages. This is mostly required to comply some interfaces
+// between mtglib and its internals to avoid circular dependencies.
+//
+// This package should contain only bare minimum and mostly technical.
+package essentials