Add 'missing features' docs (#176)

* add 'missing features' docs

* add 'URL Trailing Slash' references

* add 'Microdata Markup' note

README.md

@@ -645,3 +645,48 @@ SEOTools::setDescription($description);
 SEOTools::setCanonical($url);
 SEOTools::addImages($urls);
 ```
+
+Missing Features
+----------------
+
+There are many SEO-related features, which you may need for your project. While this package provides support for the basic ones,
+other are out of its scope. You'll have to use separated packages fot their integration.
+
+### SiteMap
+
+This package does not support sitemap files generation. Please consider usage one of the following packages for it:
+
+- [laravelium/sitemap](https://packagist.org/packages/laravelium/sitemap)
+
+- [spatie/laravel-sitemap](https://packagist.org/packages/spatie/laravel-sitemap)
+
+### URL Trailing Slash
+
+This package does not handle URL consistency regardless absence or presence of the slash symbol at its end.
+Please consider usage one of the following packages if you need it:
+
+- [illuminatech/url-trailing-slash](https://packagist.org/packages/illuminatech/url-trailing-slash)
+
+- [fsasvari/laravel-trailing-slash](https://packagist.org/packages/fsasvari/laravel-trailing-slash)
+
+### Microdata Markup
+
+This package does provide generation of the [microdata HTML markup](https://www.w3.org/TR/microdata/). If you need to create HTML like the following one:
+
+```html
+<div itemscope>
+ <p>My name is
+  <span itemprop="name">Elizabeth</span>.</p>
+</div>
+```
+
+you will need to handle it yourself.
+
+> Note: nowadays microdata markup is considered to be outdated. It is recommened to use [JSON Linked Data](https://json-ld.org/) instead,
+  which is supported by this extension.
+
+### RSS
+
+This package does not support RSS feed generation or related meta data composition. Please consider usage one of the following packages for it:
+
+- [spatie/laravel-feed](https://packagist.org/packages/spatie/laravel-feed)