add: doc photodown

doc/photodown/gallery.md

@@ -0,0 +1,53 @@
+# Block
+
+## Usage
+
+```
++++++
+config
+---
+style
+---
+content
++++++
+```
+
+### Config
+
+| type |
+| --- |
+| image-list |
+
+| direction |
+| --- |
+| down |
+| up |
+| left |
+| right |
+
+
+
+### Style
+Just css
+
+
+### Content
+
+```
+![img1]
+description 1
+--
+![img2]
+description 2
+--
+...
+```
+
+| name | usage |
+| --- | --- |
+| style | Your custom css, multiline supported |
+| content | Your bozodown | 
+
+## Effect
+
+Create a group of image