add fig installation link

gunstage is available on Fig¹ and shell plugins often include a link to Fig in
their installation instructions; this pull request fixes #123 and is related
to #110 and #122.

1. https://fig.io/plugins/other/gunstage_LucasLarson

Signed-off-by: Lucas Larson <LucasLarson@riseup.net>

readme.adoc

@@ -95,8 +95,9 @@ Installation
 ^^^^^^^^^^^^
 `gunstage` can be
 https://gist.github.com/06009589d7887617e061481e22cf5a4a[installed as a
-plugin^] or you can just clone this repository, run the following command, and
-then restart your terminal:
+plugin^], https://fig.io/plugins/other/gunstage_LucasLarson[installed
+using Fig^], or you can just clone this repository, run the following
+command, and then restart your terminal:
 [source,zsh]
 $ printf '%s\n' '. /path/to/gunstage.plugin.zsh' \
   >>"${HOME-}"'/.'"${SHELL##*[-./]}"'rc'