Documentation for other comment engines

Related to #82

docs/comments.md

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+# Adding comments to your blog
+
+## Disqus
+
+Alchemy supports Disqus comments by default. Specify `DISQUS_SITENAME`
+in your `pelicanconf.py` to enable comments integration.
+
+
+## Other comment engines
+
+Other comment engines may be integrated into Alchemy by overriding default
+template for `include/comments.html`:
+
+- Check the documentation for your comment provider and save the required HTML
+  snippet as `any_directory/include/comments.html`. Example snippets for some
+  comment engines are listed below.
+- Set [THEME_TEMPLATES_OVERRIDES] to contain correct path to templates
+  directory:
+
+```python
+THEME_TEMPLATES_OVERRIDES = ['any_directory']  # paths are relative to location of pelicanconf.py
+```
+
+[THEME_TEMPLATES_OVERRIDES]: https://docs.getpelican.com/en/stable/settings.html?highlight=THEME_TEMPLATES_OVERRIDES#themes
+
+
+## Integration snippets for different comment engines
+
+#### [isso](https://github.com/posativ/isso)
+
+```html
+<div id="isso_thread">
+  <script data-isso="{{ ISSO_URL }}/" src="{{ ISSO_URL }}/js/embed.min.js"></script>
+  <section id="isso-thread"></section>
+</div>
+```
+
+> ISSO_URL may be provided via pelicanconf.py or replaced by hardcoded value
+> in the template