Created User Login API (markdown)

User-Login-API.md

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+# User Login API
+
+_Last Updated 2017-04-22_
+
+## `POST /api/login`
+
+Takes a `application/json` payload.
+
+### Request JSON
+
+```
+{
+  email : <login email>,
+  password : <login password>
+}
+```
+
+### Response JSON
+
+#### Success
+
+Response code of `200 OK`
+
+```
+{
+  success : true,
+  session_key : <session_key>
+}
+```
+
+#### Failure
+
+If something is missing from the request, or is invalid, you will get a response code of `400 BAD REQUEST` and the following:
+
+```
+{
+  success : false,
+  message : <error message>
+  error : <error type>
+}
+```
+
+If the username or email was incorrect, you will get a `401 UNAUTHORISED` and the following:
+
+```
+{
+  success : false,
+  message : 'Email or password is invalid',
+  error : 'login_fail'
+}
+
+If there was a server error, you will get a `500 SERVER ERROR` and possibly the following:
+
+```
+{
+  success : false,
+  message : 'An unknown error occurred when logging in'
+  error : 'server_error'
+}
+```