Make AsciiDoc linguist-detectable

Before this change, project gets detected as:

Shell 32.9%
HTML 21.1%
Dockerfile 16.7%
Python 14.4%
Makefile 6.9%
XSLT 4.6%
CSS 3.4%

After this change, project gets detected as:

AsciiDoc 95.8%
Shell 1.4%
HTML 0.9%
Dockerfile 0.7%
Python 0.6%
Makefile 0.3%
Other 0.3%

This change was added for a cosmetic effect on GitHub.

.gitattributes

@@ -0,0 +1 @@
+*.asciidoc linguist-detectable