cmd/pprof: update README to explain relationship with github.com/google/pprof

For google/pprof#132.

Change-Id: I73785bc8662565ff7b2fac66e23497dd06fc25b6
Reviewed-on: https://go-review.googlesource.com/40231
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/pprof/README

-The pprof in this directory is adapted from the pprof used inside Google
-for C++, Java, and Go programs. Because it was developed for that broader
-context, it is overgeneralized when used here for the specific use case
-of profiling standard Go programs. However, we've left the abstractions
-intact in order to share updates between this copy and Google's internal one.
+This directory is the copy of Google's pprof shipped as part of the Go distribution.
+The bulk of the code is vendored from github.com/google/pprof and is in
+../vendor/github.com/google/pprof.
 
+Two important notes:
+
+1. Using github.com/google/pprof directly (for example, after installing
+with "go get") should work with Go programs, but we cannot guarantee that.
+What we test is that the "go tool pprof" shipped with each Go release works
+with programs from that release.
+
+2. Pprof is used inside Google for C++, Java, and Go programs.
+Because it was developed for that broader context, it is overgeneralized
+when used here for the specific use case of profiling standard Go programs.
+However, we've left the abstractions intact in order to share updates
+between our vendored copy and Google's internal one.
 Please do not take the level of abstraction in this program as an example
 to follow in your own.