Update mongoose docs

PUBLISHED_FROM=cfd0018a5824c5ccac532558be86c17cda64ce50

docs/http/digest_auth.md

@@ -2,4 +2,15 @@
 title: Digest Authentication
 ---
 
-TBD
+Mongoose has a built-in Digest (MD5) authentication support. In order to
+enable Digest authentication, create a file `.htpasswd` in the directory
+you would like to protect. That file should be in the format that Apache's
+`htdigest` utility.
+
+You can use either Apache `htdigest` utility, or
+Mongoose pre-build binary (https://www.cesanta.com/products/binary)
+to add new users into that file:
+
+```
+mongoose -A /path/to/.htdigest mydomain.com joe joes_password
+```