doc: Add a note to imgtool docs
Because of branch, and https server configuration issues, the plain 'go
get' instructions to build imgtool don't work.  Add a note to the README
file for imgtool and give some instructions on how to manually clone the
necessary git repositories.
Image Manipulation Tool
#######################
+.. note:: Special installation instructions:
+ Due to some configuration issues with servers, it may not be
+ possible to build imgtool with a simple ``go get`` command. It is
+ possible to build the tool with the following steps::
+
+ # Set GOPATH to a work dir
+ $ mkdir -p $GOPATH/src/mynewt.apache.org
+ $ mkdir -p $GOPATH/src/github.com
+ $ cd $GOPATH/src/mynewt.apache.org
+ $ git clone -b develop https://github.com/apache/incubator-mynewt-newt newt
+ $ cd $GOPATH/src/github.com
+ $ git clone https://github.com/runtimeco/mcuboot
+ $ go get github.com/runtimeco/mcuboot/imgtool
+
+ At this point, there should be an ``imgtool`` binary in $GOPATH/bin
+
+.. note:: Go version:
+ Although many distributions include the Go tools, these are usually
+ fairly out of date. This code has been developed and tested with
+ Go version 1.7.x. For best results, follow the `download
+ instructions`_ to install a recent version.
+
+.. _download instructions: https://golang.org/dl/
+
Introduction
============
