doc: add a note about loading profile files

If one is somewhat new to the command line or shell, it might be
surprising that changes applied to a file like $HOME/.profile will
seemingly not take effect, even if new shells are started.

Add a note about how shells usually only load these when the user logs
into a machine, to minimize the amount of people stuck and confused by
this.

Fixes golang#24756.

Change-Id: Ic68d8c97933f3f080b151a107633ecad76a163a4
Reviewed-on: https://go-review.googlesource.com/105557
Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

doc/install.html

@@ -106,6 +106,12 @@ <h3 id="tarball">Linux, Mac OS X, and FreeBSD tarballs</h3>
 export PATH=$PATH:/usr/local/go/bin
 </pre>
 
+<p>
+<b>Note</b>: changes made to a <code>profile</code> file may not apply until the
+next time you log into your computer. Alternatively, you can apply them to the
+current shell with a command like <code>source $HOME/.profile</code>.
+</p>
+
 <h4 id="tarball_non_standard">Installing to a custom location</h4>
 
 <p>