Update .gitattributes file.

Author: Maikuolan

.gitattributes

@@ -7,12 +7,13 @@
 # https://github.com/github/linguist
 
 *.md linguist-documentation
+*.txt linguist-documentation
 
 # Stuff not needed for dist installs. Refer to:
 # https://git-scm.com/docs/gitattributes#_creating_an_archive
 # https://getcomposer.org/doc/06-config.md#preferred-install
 
-/.github export-ignore
-/_docs export-ignore
 /.gitattributes export-ignore
+/.github export-ignore
 /.gitignore export-ignore
+/_docs export-ignore