Make AsciiDoc linguist-detectable

Before this change, project gets detected as:

Python 33.8%
HTML 28.7%
C++ 16.7%
Go 11.1%
CSS 8.6%
JavaScript 1.1%

After this change, project gets detected as:

AsciiDoc 94.9%
Python 1.7%
HTML 1.5%
C++ 0.8%
Go 0.6%
CSS 0.4%
JavaScript 0.1%

This change was added for a cosmetic effect on GitHub.

.gitattributes

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