Add access tokens (#602)

docs/usage/access-tokens.md

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+# Access tokens
+
+Access tokens are used to access your private gists and their raw content. For now, it is the only use while a future API is being developed.
+
+## Creating an access token
+
+To create an access token, follow these steps:
+1. Go to Settings
+2. Select the "Access Tokens" menu
+3. Choose a name for your token, the scope and an expiration date (optional), then click "Create Access Token"
+
+## Using an access token
+
+Once you have created an access token, you can use it to access your private gists with it.
+
+Replace `<token>` with your actual access token in the following examples.
+
+```shell
+# Access raw content of a private gist, latest revision for "file.txt". Note: this URL is obtained from the "Raw" button on the gist page.
+curl -H "Authorization: Token <token>" \
+ http://opengist.example.com/user/gist/raw/HEAD/file.txt
+
+# Access the JSON representation of a private gist. See "Gist as JSON" documentation for more details.
+curl -H "Authorization: Token <token>" \
+ http://opengist.example.com/user/gist.json
+```