adds demo gif. switches readme to markdown.

fixes #21

readme.md

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+# highlight-thing.el
+
+Global minor mode to highlight the thing under point.
+
+[![MELPA](https://melpa.org/packages/highlight-thing-badge.svg)](https://melpa.org/#/highlight-thing)
+
+Similar to `highlight-symbol-mode`, but does not rely on font-lock
+functionality and does not provide functionality to navigate to different
+occurrences of the current symbol under point.
+
+No external dependencies, uses `thingatpt` and `hi-lock` functionality that is
+included with GNU Emacs.
+
+![Demo](demo.gif)
+
+## Installation
+
+Basic setup:
+
+```emacs-lisp
+  (require 'highlight-thing)
+  (global-highlight-thing-mode)
+```
+
+Alternatively you can use the buffer-local version:
+
+```emacs-lisp
+  (add-hook 'prog-mode-hook 'highlight-thing-mode)
+```
+
+The default is to highlight the symbol under point, but you can customize
+`hightlight-thing-what-thing` to highlight different components. Set the following to only
+highlight the word under point:
+
+```emacs-lisp
+  (setq highlight-thing-what-thing 'word)
+```
+
+As an extension to the `thing-at-point` capabilities you can select `region` in
+which case an active region is used as the thing to highlight (cf. [#7](https://github.com/fgeller/highlight-thing.el/issues/7)).
+
+[@antoineB](https://github.com/antoineB) provided the functionality to use a custom regex function for a
+thing-at-point - thanks! For his use case, cf. [#18](https://github.com/fgeller/highlight-thing.el/pull/18). To use it store the custom regex function as the
+property `highlight-thing-regex-fn` on your `thing-at-point`:
+
+```emacs-lisp
+  (put 'word 'highlight-thing-regex-fn 'your-regex-fn)
+```
+
+Customize the face `hi-yellow` to change the colors that are used for
+highlighting.
+
+You can customize the delay before this mode attempts to highlight the thing
+under point. For example, push it out to 1.5 seconds from the default 0.5:
+
+```emacs-lisp
+  (setq highlight-thing-delay-seconds 1.5)
+```
+
+You can limit the highlighting of things to the current defun via the
+following setting:
+
+```emacs-lisp
+  (setq highlight-thing-limit-to-defun t)
+```
+
+You can configure the matching of occurrences to be case-sensitive via the following setting:
+
+```emacs-lisp
+  (setq highlight-thing-case-sensitive-p t)
+```
+
+If you want all the matches highlighted but not the one occurrence
+at the point itself, you can do so by:
+
+```emacs-lisp
+  ;; Don't highlight the thing at point itself. Default is nil.
+  (setq highlight-thing-exclude-thing-under-point t)
+```
+
+If you want to highlight the current region when active or thing at point
+when inactive:
+
+```emacs-lisp
+  (setq highlight-thing-prefer-active-region t)
+```
+
+If you want to prevent highlighting certain strings:
+
+```emacs-lisp
+  (setq highlight-thing-ignore-list '("False" "True"))
+```
+
+If you want to highlight in all visible buffers:
+
+```emacs-lisp
+  (setq highlight-thing-all-visible-buffers-p t)
+```
+
+If you want to restrict the highlighting to lines surrounding points, e.g. to
+reduce the load of highlighting in large buffers as in [#16](https://github.com/fgeller/highlight-thing.el/issues/16), consider
+customizing the following variables:
+
+```emacs-lisp
+  (setq highlight-thing-limit-to-region-in-large-buffers-p nil
+        highlight-thing-narrow-region-lines 15
+        highlight-thing-large-buffer-limit 5000)
+```