go.tools/README: update to make truthful

The old README was misleading, saying the packages were not of interest.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/14516051

README

@@ -1,7 +1,10 @@
 This subrepository holds the source for various packages and tools that support
 the Go programming language.
 
-Although at least some of the tools themselves will be included in binary Go distributions,
-the packages from which they are built are of little interest to most Go programmers.
+Some of the tools, godoc and vet for example, are included in binary Go distributions.
+Others, including the Go oracle and the test coverage tool, can be fetched with "go get".
+
+Packages include a type-checker for Go and an implementation of the Single Static
+Assignment (SSA) representation for Go programs.
 
 To submit changes to this repository, see http://golang.org/doc/contribute.html.