Fix: move files to locations other people expect them

LICENSE, README, and CHANGELOG files normally are in the root
these days. This also means GitHub picks up on them a bit easier.

Author: TrueBrain

docs/license.txt LICENSE

@@ -13,9 +13,9 @@ PROJECT_FILENAME    ?= opengfx
 OBG_FILENAME        ?= $(PROJECT_FILENAME).obg
 
 # Documentation files
-LICENSE_FILE   ?= docs/license.txt
-CHANGELOG_FILE ?= docs/changelog.txt
-README_FILE    ?= docs/README.md
+LICENSE_FILE   ?= license.txt
+CHANGELOG_FILE ?= changelog.txt
+README_FILE    ?= README.md
 
 # Define these if you do not want to install the doc files along the grf files. Mind, that doing so
 # will cause OpenTTD to not be able to show them properly ingame, thus the user will likely not be