Tell Git how to check Markdown (.md) source files

Disallow tabs in document indentation.  We will use four spaces instead
of a tab to indent nested blocks.

Use large conflict markers because otherwise '=======' underlines in
Markdown section headers look like conflict markers to the automatic
checks.

.gitattributes

@@ -21,3 +21,5 @@ pre-commit       crlf=input
 *.cmake          whitespace=tab-in-indent,-blank-at-eol
 
 *.c              hooks.MaxObjectKiB=0
+
+*.md             whitespace=tab-in-indent conflict-marker-size=30