FEM: code conventions, add some text for LF line endings

src/Mod/Fem/coding_conventions.md

@@ -20,7 +20,9 @@ These coding rules apply to FEM module code only. Other modules or the base syst
 ## Python and C++
 ### Code formatting
 - All files should have a license header
-- Unix line endings are preferred
+- Unix line endings will be used:
+    - a .gitattributes file has been added to ensure line endings of text files are LF
+    - use `?w=1` in link address to surpress line ending changes on github
 - never use mixed line endings on one file
 - 4 Spaces for indent
 - no trailing white spaces