Added documentation for louislam/uptime-kuma#2558

Signed-off-by: Matthew Nickson <mnickson@sidingsmedia.com>

APIKeys.md

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+# API Keys
+
+API keys can be used to access secured HTTP APIs, including the
+Prometheus metrics endpoint.
+
+## Adding an API key
+
+Management of API keys is done through the API Key management page,
+accessible from the drop down menu, just below the maintenance link. This
+page will show you all of your API keys, including expired keys. The
+green button in the top left can be used to add a new key.
+
+You can set the name of the key, as well as it's expiry date, or
+optionally set the key to never expire. When you click generate, you
+will be shown the API key in a pop up dialog, you should make sure to
+make a copy of the key as it will not be shown again. The API key cannot
+be used to access the web interface but you should treat API keys like
+you treat your passwords as they can still be used to access potentially
+sensitive data.
+
+Note: whilst basic authentication will continue to work after you
+update, you will be unable to use basic authentication as soon as you
+add your first API key. After you have added your first API key, basic
+authentication for endpoints will be permanently disabled.
+
+## Disabling keys
+
+It is possible to temporarily disable keys. This can be done through the
+management page. This will prevent the key from being used for
+authentication until it is enabled again.
+
+## Expired keys
+
+If a key has expired, it will not be automatically deleted. This is to
+aid in debugging systems that suddenly stop working. You can delete an
+expired key, or any key for that matter, by simply hitting the red
+delete button from the management page.
+
+## Authenticating using an API key
+
+Authentication is done by passing the API key in the `Authorization`
+header. For example, here is a request made with curl to the `metrics`
+endpoint.
+
+```
+curl -u":<key>" uptime.kuma/metrics
+```
+
+Note, the `:` is required before the key as basic authentication
+requires a username and password separated by a `:`, however we don't
+make use of the username field.
+
+Here is an example config for Prometheus:
+
+```
+  - job_name: 'uptime'
+    scrape_interval: 30s
+    scheme: http
+    static_configs:
+      - targets: ['uptime.url']
+    basic_auth: 
+      password: <api key>
+```
+
+Note: we don't need to set a password field as it is not used.