formalize trailer tag into XMLNS

docs/1.0.md

@@ -266,3 +266,44 @@ Please do not exceed `32 characters` for the display attribute.
 `<podcast:episode display="Ch.3">204</podcast:episode>`
 
 `<podcast:episode display="Day 5">9</podcast:episode>`
+
+<br><br>
+
+
+## Trailer
+`<podcast:trailer>`
+This element is used to define the location of an audio or video file to be used as a trailer for the entire podcast or a specific season.  There can be more than one trailer present in the channel of the
+feed.  This element is basically just like an `<enclosure>` with the extra `pubdate` and `season` attributes added.
+
+#### Parent
+`<channel>`
+
+#### Count
+Multiple
+
+#### Node Value
+The node value is a string, which is the title of the trailer. It is required.
+
+#### Attributes
+ - **url:** (required) This is a url that points to the audio or video file to be played. This attribute is a string.
+ - **pubdate:** (required) The date the trailer was published. This attribute is an RFC2822 formatted date string.
+ - **length:** (recommended) The length of the file in bytes. This attribute is a number.
+ - **type:** (recommended) The mime type of the file. This attribute is a string.
+ - **season:** (optional) If this attribute is present it specifies that this trailer is for a particular season number. This attribute is a number.
+
+If there is more than one trailer tag present in the channel, the most recent one (according to it's `pubdate`) should be chosen as the preview by default within podcast apps.  If the `season` attribute is present, it must
+be a number that matches the format of the `<podcast:season>` tag.  So, for a podcast that has 3 published seasons, a new `<podcast:trailer season="4">` tag can be put in the channel to later be matched up with a `<podcast:season>4<podcast:season>`
+tag when it's published within a new `<item>`.
+
+#### Examples
+```xml
+<podcast:trailer pubdate="Thu, 01 Apr 2021 08:00:00 EST" url="https://example.org/trailers/teaser" length="12345678" type="audio/mp3">Coming April 1st, 2021</podcast:trailer>
+```
+
+```xml
+<podcast:trailer pubdate="Thu, 01 Apr 2021 08:00:00 EST" url="https://example.org/trailers/season4teaser" length="12345678" type="video/mp4" season="4">Season 4: Race for the Whitehouse</podcast:trailer>
+
+(later matches with)
+
+<podcast:season name="Race for the Whitehouse">4</podcast:season>
+```