Update USAGE.md; add changelog entry

CHANGELOG.md

@@ -4,6 +4,9 @@ Since v0.10.0 this project adheres to [Semantic Versioning](http://semver.org/)
 
 ### Upcoming version
 
+#### Added
+* New method `Wikimate::logout()` ([#124])
+
 #### Changed
 
 * Updated `Wikimate::token()` to remember CSRF token and reduce API calls ([#122])
@@ -146,3 +149,4 @@ Since v0.10.0 this project adheres to [Semantic Versioning](http://semver.org/)
 [#118]: https://github.com/hamstar/Wikimate/pull/118
 [#121]: https://github.com/hamstar/Wikimate/pull/121
 [#122]: https://github.com/hamstar/Wikimate/pull/122
+[#124]: https://github.com/hamstar/Wikimate/pull/124

USAGE.md

@@ -339,7 +339,7 @@ Both methods return an array of the MediaWiki API result.
 API requests are made over HTTP with a user agent string to identify
 the client to the server. By default the user agent is formatted as:
 
-`Wikimate <VERSION> (https://github.com/hamstar/Wikimate)`
+`Wikimate/<VERSION> (https://github.com/hamstar/Wikimate)`
 
 The string can be retrieved and customized via:
 
@@ -362,6 +362,7 @@ print_r($page->getError());
 
 For MediaWiki API errors, the array contains the 'code' and 'info' key/value pairs [defined by the API](https://www.mediawiki.org/wiki/API:Errors_and_warnings#Errors).  For other errors, the following key/value pairs are returned:
 * 'login' for Wikimate authentication problems
+* 'auth' for Wikimate authentication problems
 * 'token' for Wikimate token problems
 * 'page' for WikiPage errors
 * 'file' for WikiFile errors