doc: Import documentation from wiki

Signed-off-by: Florian Pritz <bluewind@xinu.at>

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+# API
+
+The API provides programmatic access to upload, delete files, view the
+currently uploaded ones and combine them to multipastes, as well as functions
+to manage api keys. Responses are available in JSON.
+
+## General notes
+
+### URLs
+
+The URLs for API endpoints start with the base URL followed by `/api/`, the
+version supported by the client and the endpoint. For example:
+`https://paste.xinu.at/api/v1.0.0/some/endpoint`.
+
+The version number follows the [semantic versioning guidelines](http://semver.org/).
+The requested version number must be of the format `vX[.X[.X]]` with X being
+a positive number. `v1` and `v1.0` will both be treated as `v1.0.0`.
+
+The most recent API version is `v2.1.0`.
+
+### Compatibility
+
+The API will evolve by adding and removing endpoints, parameters and keywords.
+Unknown keywords should be treated gracefully by your application.
+
+Behavior not documented here should not be expected to be available and may be
+changed without notice.
+
+### Unless stated otherwise ...
+
+
+*  ... requests should be sent via POST.
+*  ... endpoints expect an apikey with access level `apikey` to be sent with the request.
+*  ... requests should not be assumed to be atomic (i.e. data may be changed on the server despite an error being returned).
+
+
+*  ... timestamps are returned as UNIX timestamps (seconds).
+*  ... sizes are returned in bytes.
+*  ... values are specific to the user owning the apikey (e.g. the `total_size` field of file/history).
+
+
+*  ... errors will generate a response with `status=error`.
+*  ... error messages may differ from those listed in the tables.
+*  ... errors listed are only the most common ones (i.e. the lists are non-exhaustive).
+
+### Access levels
+
+An api key can have one of the following access levels. Levels further down in
+the table include those above themselves.
+
+ | access-level | Comment                                       | 
+ | ------------ | -------                                       | 
+ | basic        | Allows only uploading of files                | 
+ | apikey       | Allows to delete uploads and view the history | 
+ | full         | Allows everything                             | 
+
+## General response structure
+
+Replies will always contain a `status` field which can contain the following
+values: `success`, `error`. If the reply does not contain the status field
+it should be regarded as invalid.
+
+If `status=success` the reply will contain a `data` field which contains
+function specific data.
+
+### Error handling
+
+If `status=error` the reply will be of the following format:
+
+```javascript
+// Error response format
+responseError = {
+	"status": "error",
+	"message": "A message that can be displayed to the user",
+	"error_id": "program/useable/error/id",
+	"data": object or array, // optional, only used if mentioned
+}
+```
+
+
+## General errors
+
+These are the most common errors that can be returned by any API call.
+
+ | error_id                     | Message                                    | Note                      | 
+ | --------                     | -------                                    | ----                      | 
+ | api/invalid-version          | Invalid API version requested              | Failed syntax check       | 
+ | api/invalid-endpoint         | Invalid endpoint requested                 | Failed syntax check       | 
+ | api/version-not-supported    | Requested API version is not supported     |                           | 
+ | api/unknown-endpoint         | Unknown endpoint requested                 | Likely a typo in your URL | 
+ | internal-error               | An unhandled internal server error occured |                           | 
+ | user/api-login-failed        | API key login failed                       |                           | 
+ | api/insufficient-permissions | Access denied: Access level too low        |                           | 
+ | api/not-authenticated        | Not authenticated                          | Likely no apikey was sent | 
+
+## Endpoints
+
+All return values below assume success unless stated otherwise and are the
+content of the `data` field mentioned above.
+
+```javascript
+// General definitions
+
+// Listed above
+access-level: String;
+
+// An api key
+apikey: String;
+
+// An ID that can be used to display a multipaste or a single file
+upload-id: String;
+
+
+// General success response
+responseSuccess = {
+	"status": "success",
+	"data": object or array,
+}
+```
+
+For the examples set the following variables in your shell. Please note that
+they and their values will show up in your shell history and in top/ps if used.
+Be careful on untrusted systems.
+
+```bash
+apikey="anApiKey"
+base="https://paste.xinu.at/api/v2.0.0"
+```