update custom tags doc

docs/05-custom-tags.md

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+# Custom tags
+
+Marko gives you access to the same API that is used to write the [Core Tags]() so that you can extend the language with custom tags and attributes.
+
+> **PROTIP:** We recommend that custom tags and custom attributes have at least one dash to indicate that they are not part of the standard HTML grammar.
+
+## Writing custom tags
+
+To get started let's look at template-based tags which allow you to include another template using a named custom tag rather than specifying a filesystem path and using `<include>`.
+
+### Discovering tags
+
+When compiling a template Marko will search starting at template's directory, up to the project root for directories named `components/`. It then attempts to load the children of these directories as custom tags.  The children can be a Marko template or a directory with an `index.marko` template (and other supporting files).
+
+```dir
+components/
+    app-header/
+        index.marko
+        logo.png
+        style.css
+    app-footer.marko
+pages/
+    home/
+        components/
+            home-banner.marko
+        index.marko
+```
+
+When compiling the template at `pages/home/index.marko`, the following tags would be found:
+
+- `<app-header>`
+- `<app-footer>`
+- `<home-banner>`
+
+So now, instead of needing to specify a path:
+
+```xml
+<include('../../components/app-header/index.marko')/>
+```
+
+You can just use the tag name:
+```xml
+<app-header/>
+```
+
+See [Advanced Tag Discovery Docs]() for more details on how to customize tag discovery.
+
+## Exporting tags
+
+## Using tags from npm
+
+Using custom tags from `npm` is easy.  Ensure that the package is installed and listed in your `package.json` dependencies:
+
+```
+npm install --save some-third-party-package
+```
+
+And that's it.  Marko will now discover these tags when compiling your templates and you can simply use them in your templates:
+
+```xml
+<div>
+    <some-third-party-tag/>
+</div>
+```