Update docs

README.md

@@ -38,6 +38,7 @@
 - [Advanced Options](#advanced-options)
   - [Add Origin](#add-origin)
   - [Add All Origins](#add-all-origins)
+  - [Exclude Domains](#exclude-domains)
   - [Header Detection](#header-detection)
   - [Path Matching](#path-matching)
   - [Remove Origin](#remove-origin)
@@ -155,6 +156,16 @@ Alternatively you can use the `Allow All` button.
 
 > Note that the remote origins should either provide a valid HTTP header (see [Header Detection](#header-detection)) and/or valid URL path (see [Path Matching](#path-matching)). Otherwise you'll have to add the origin explicitly and set its [Path Matching](#path-matching) regular expression.
 
+## Exclude Domains
+
+When you enable the extension for all origins, a new `Excluded Domains` section will become available in the options page.
+
+When the extension is enabled for all origins, it will try to inject itself into all pages with URLs ending with markdown file extension using the [Path Matching](#path-matching) for the `*://*` origin.
+
+However a few websites are known to serve pages with URLs ending with markdown file extension, but instead of a raw markdown they serve a rendered HTML there. For example: https://github.com/simov/markdown-viewer/blob/master/README.md
+
+When you enable the extension for all origins you get a list of domains excluded by default: `bitbucket.org`, `getcomposer.org`, `github.com`, `gitlab.com`, however you can extend/modify that list as you want.
+
 ## Header Detection
 
 When this option is enabled the extension will check for the `text/markdown` and `text/x-markdown` *content-type* header before trying to match the path: