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

@@ -14,4 +14,6 @@ pre-commit       crlf=input
 *.txt            whitespace=tab-in-indent
 *.cmake          whitespace=tab-in-indent
 
+*.md             whitespace=tab-in-indent conflict-marker-size=30
+
 /ThirdParty/sqlite/vtksqlite/vtk_sqlite3.c hooks.MaxObjectKiB=4096