Update docs

docs/configuration.md

@@ -44,6 +44,7 @@ echo $(xxd -g 2 -l 64 -p /dev/random | tr -d '\n')
 * `IMGPROXY_USER_AGENT`: the User-Agent header that will be sent with the source image request. Default: `imgproxy/%current_version`
 * `IMGPROXY_USE_ETAG`: when set to `true`, enables using the [ETag](https://en.wikipedia.org/wiki/HTTP_ETag) HTTP header for HTTP cache control. Default: `false`
 * `IMGPROXY_ETAG_BUSTER`: change this to change ETags for all the images. Default: blank
+* `IMGPROXY_USE_LAST_MODIFIED`: when set to `true`, enables using the [Last-Modified](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified) HTTP header for HTTP cache control. Default: `false`
 * `IMGPROXY_CUSTOM_REQUEST_HEADERS`: ![pro](./assets/pro.svg) list of custom headers that imgproxy will send while requesting the source image, divided by `\;` (can be redefined by `IMGPROXY_CUSTOM_HEADERS_SEPARATOR`). Example: `X-MyHeader1=Lorem\;X-MyHeader2=Ipsum`
 * `IMGPROXY_CUSTOM_RESPONSE_HEADERS`: ![pro](./assets/pro.svg) a list of custom response headers, separated by `\;` (can be redefined by `IMGPROXY_CUSTOM_HEADERS_SEPARATOR`). Example: `X-MyHeader1=Lorem\;X-MyHeader2=Ipsum`
 * `IMGPROXY_CUSTOM_HEADERS_SEPARATOR`: ![pro](./assets/pro.svg) a string that will be used as a custom header separator. Default: `\;`