add explanation of restic automation

every time i look at restic, i block on this and figured it may be useful for others

doc/conf.py

@@ -19,7 +19,7 @@ import os
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = []
+extensions = ['sphinx.ext.extlinks']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -104,3 +104,7 @@ html_static_path = ['_static']
 
 # Output file base name for HTML help builder.
 htmlhelp_basename = 'resticdoc'
+
+extlinks = {
+    'issue': ('https://github.com/restic/restic/issues/%s', '#'),
+}