Add documentation for the "negative ignore pattern" feature

docs/config.md

@@ -169,6 +169,12 @@ Make sure to quote those when using wildcards in the config file.
 
 Patterns used on a specific dotfile can be specified relative to the dotfile destination (`dst`).
 
+Similar to a `.gitignore` file, you can prefix ignore patterns with an exclamation point (`!`).
+This so-called "negative ignore pattern" will cause any files that match that pattern to __not__ be ignored,
+provided they *would have* been ignored by an earlier ignore pattern (dotdrop will warn if that is not the
+case). This feature allows you to, for example, ignore all files within a certain directory, except for one
+particular one (see example below). 
+
 ```yaml
 config:
   cmpignore:
@@ -216,3 +222,14 @@ config:
   - "testdir"
 ...
 ```
+
+To ignore all files within a certain directory relative to `dst`, except one called `custom_plugin.zsh`:
+```yaml
+dotfiles:
+  d_zsh:
+    src: zsh
+    dst: ~/.config/zsh
+	impignore:
+    - "plugins/*"
+    - "!plugins/custom_plugin.zsh"
+```

tests-ng/negative-ignore-no-match.sh

@@ -104,7 +104,8 @@ rm -rf ${tmpd}
 echo "[+] install with negative ignore in dotfile"
 echo '(1) expect dotdrop install to warn when negative ignore pattern does not match an already-ignored file'
 
-patt="[WARN] no files that are currently being ignored match \"*/ignore_me/c\". In order for a negative ignore pattern to work, it must match a file that is being ignored by a previous ignore pattern."
+patt="[WARN] no files that are currently being ignored match \"*/ignore_me/c\". In order for a negative ignore
+pattern to work, it must match a file that is being ignored by a previous ignore pattern."
 cd ${ddpath} | ${bin} install -c ${cfg2} --verbose 2>&1 >/dev/null | grep -F "${patt}" ||
   (echo "dotdrop did not warn when negative ignore pattern did not match an already-ignored file" && exit 1)