Update changelog and docs

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## [Unreleased]
 ### Added
 - Add `IMGPROXY_ETAG_BUSTER` config.
+- (pro) [watermark_text](https://docs.imgproxy.net/generating_the_url?id=watermark-text) processing option.
 
 ### Change
 - Improved ICC profiles handling.

docs/generating_the_url.md

@@ -430,6 +430,21 @@ When set, imgproxy will use the image from the specified URL as a watermark. `ur
 
 Default: blank
 
+### Watermark text<i class='badge badge-pro'></i> :id=watermark-text
+
+```
+watermark_text:%text
+wmt:%text
+```
+
+When set, imgproxy will generate image from the provided text and use it as a watermark. `text` is Base64-encoded text of the custom watermark.
+
+By default, text color is black and font is `sans 16`. You can use [Pango markup](https://docs.gtk.org/Pango/pango_markup.html) in the `text` value to change the style.
+
+If you want to use your custom font, you need to put it to `/usr/share/fonts` inside a contsainer.
+
+Default: blank
+
 ### Style<i class='badge badge-pro'></i> :id=style
 
 ```