Document converting `+` to `%2B` in an API query parameter

doc/api/README.md

@@ -455,6 +455,23 @@ Content-Type: application/json
 }
 ```
 
+## Encoding `+` in ISO 8601 dates
+
+If you need to include a `+` in a query parameter, you may need to use `%2B` instead due
+a [W3 recommendation]((http://www.w3.org/Addressing/URL/4_URI_Recommentations.html) that
+causes a `+` to be interpreted as a space. For example, in an ISO 8601 date, you may want to pass
+a time in Mountain Standard Time, such as:
+
+```
+2017-10-17T23:11:13.000+05:30
+```
+
+The correct encoding for the query parameter would be:
+
+```
+2017-10-17T23:11:13.000%2B05:30
+```
+
 ## Clients
 
 There are many unofficial GitLab API Clients for most of the popular