diff --git a/README.md b/README.md
index 868f852264f2ad6d2b7d4de9f47bd3b32d147151..624717da69882dbe75ba3465eeee18d97b13634c 100644
--- a/README.md
+++ b/README.md
@@ -7,8 +7,14 @@ The HTTP API is responsible for registration, password resets and SMS verificati
 
 ### `GET /authentication/$phoneNumber`
 
+Requests an SMS containing a 6 digit code to be send to `$phoneNumber`. The number is formatted according to E.164 (leading +, including the contry code, no spaces).
+
 #### Header
 
+* `Accept-Language`: ISO 639-1 two letter language code. Will be used to change the language of the SMS
+* `Installation-Id`: Randomly generated (at install time) UUID supplied by the client. Will be used for rate limiting on top of per number and per IP limits. Limits users on requesting multiple SMS for multiple phone numbers.
+* `User-Agent`: Formatted to `Name/Version` with version being formatted according to [Sementic Versioning](https://semver.org/).
+
 #### Response Codes
 
 ### `POST /password`