go-module.eclass: add a manual example of how to extract the content of EGO_SUM

Signed-off-by: William Hubbs <williamh@gentoo.org>

eclass/go-module.eclass

@@ -103,6 +103,14 @@ RESTRICT+=" strip"
 # You can use some combination of sed/awk/cut to extract the
 # contents of EGO_SUM or use the dev-go/get-ego-vendor tool.
 # 
+# One manual way to do this is the following:
+#
+# @CODE
+#
+# cat go.sum | cut -d" " -f1,2 | awk '{print "\t\"" $0 "\""}'
+# 
+# @CODE
+#
 # The format of go.sum is described upstream here:
 # https://go.dev/ref/mod#go-sum-files
 #