Fix example Makefile to avoid $GOROOT in case it has spaces.

doc/contribute.html

 <!-- Contributing to the Go project -->
 
 <h2 id="Introduction">Introduction</h2>
 
 <p>
 This document explains how to write a new package,
 how to test code, and how to contribute changes to the Go project.
 It assumes you have installed Go using the
 <a href="install.html">installation instructions</a>.  (Note that
 the <code>gccgo</code> frontend lives elsewhere;
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 <p>
 It would be nice to have Go-specific tools that
 inspect the source files to determine what to build and in
 what order, but for now, Go uses GNU <code>make</code>.
 Thus, the first file to create in a new package directory is
 usually the <code>Makefile</code>.
 The basic form is illustrated by <a href="../src/pkg/container/vector/Makefile"><code>src/pkg/container/vector/Makefile</code></a>:
 </p>
 
 <pre>
-include $(GOROOT)/src/Make.$(GOARCH)
+include ../../../Make.$(GOARCH)
 
 TARG=container/vector
 GOFILES=\
         intvector.go\
         stringvector.go\
         vector.go\
 
-include $(GOROOT)/src/Make.pkg
+include ../../../Make.pkg
 </pre>
 
 <p>
 The first and last lines <code>include</code> standard definitions and rules,
+<code>$(GOROOT)/src/Make.$(GOARCH)</code> and <code>$(GOROOT)/src/Make.pkg</code>,
 so that the body of the <code>Makefile</code> need only specify two variables.
+For packages to be installed in the Go tree, use a relative path instead of
+<code>$(GOROOT)/src</code>, so that make will work correctly even if <code>$(GOROOT)</code> contains spaces.
 </p>
 
 <p>
 <code>TARG</code> is the target install path for the package,
 the string that clients will use to import it.
 This string should be the same as the directory
 in which the <code>Makefile</code> appears, with the
 <code>$GOROOT/src/pkg/</code> removed.
 </p>
 
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 codereview.appspot.com as the email address in these files.</ol>
 <p>
 This rigamarole needs to be done only for your first submission.
 </p>
 
 <p>
 Once the code is ready to be committed,
 one of the Go developers at Google will approve and submit
 your change.
 </p>