gitattributes: default to LF

1. Default to LF line endings. Windows users should use a real text editor when
   coding.
2. Allow markdown/license files to be autoconverted so they're readable
   everywhere.

fixes #6195

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

.gitattributes

+# Default to text
+* text eol=lf
+
+# True text
+*.md text eol=auto
+LICENSE text eol=auto
+
+# Binary assets
 assets/init-doc/*          binary
 core/coreunix/test_data/** binary