Using GGG: instruct to drop description for single-commit PRs

For single commit PRs, GitHub prepends the commit message to the PR
description, before the PR template.

Contributors will then (hopefully) remove the template and keep the rest
of the description. Since the PR description is added as an in-patch
commentary for single-commit PRs, the same text appears twice in the
generatred email (once in the commit messge and once in the in-patch
commentary), decreasing the signal-to-noise ratio for reviewers.

Let's instruct readers to empty the PR description for single commit
PRs.

This partly addresses:
gitgitgadget/gitgitgadget#340

Author: phil-blain

index.html

@@ -103,6 +103,7 @@ <h2>How can you use GitGitGadget?</h2>
 				A few things to note about a GitGitGadget Pull Request:
 				<ul>
 					<li>Please make sure to use a descriptive title and description; GitGitGadget will use these as the subject and body of the cover letter (check out the <a href="https://git-scm.com/docs/MyFirstContribution#cover-letter">MyFirstContribution</a> tutorial if you are not familiar with this terminology).</li>
+					<li>If your pull request consist of a single commit, leave the pull request description empty, as your commit message itself should be enough to explain the rationale for your changes.</li>
 					<li>You can CC potential reviewers by adding a footer to the PR description with the following syntax:</br>
 					<code>CC: Revi Ewer &lt;revi.ewer@some.domain&gt;, Ill Takalook &lt;ill.takalook@other.domain&gt;</code>
 					</li>