add BOM help, see AdvancedPhotonSource/GSAS-II#259.

Author: briantoby

MDhelp/docs/mainmenu.md

@@ -85,6 +85,8 @@ resonant (if any) neutron scattering lengths for all isotopes of a selected elem
 A special set of modules, known as importers, is used to read information into GSAS-II 
 from external files. The importers that are found by GSAS-II at runtime are used to configure the import menus. Import modules are usually fairly easy to create and thus new formats are relatively easy to support. See the [GSAS-II Import Modules](https://gsas-ii.readthedocs.io/en/latest/imports.html) section of the Programmers documentation for more information on this. Note that most menus include a "**guess format from file**" option. This attempts to determine the file format by matching the file extension and a cursory examination of the file contents to the importer(s) to be tried. On occasion, this command may not succeed in correctly determining a file format. If it fails, retry by selecting the correct format from the list.
 
+While we are not able to reproduce this, with some locale settings (particularly CJK ones?), GSAS-II will not be able to read UTF-8 files that have a BOM (Byte Order Mark). If you find that you are unable to read a data file in GSAS-II, you may wish to try stripping the BOM from the file. 
+
 * **Image** - Read in one or more 2D powder diffraction images (multiple patterns can be selected). A submenu appears with choices for import of data. Each entry when selected with the mouse shows further submenus with specific imports that are available. Any of these files can be accessed from a zip file. GSAS-II can read many different image file formats including MAR345 files, Quantum ADSC files, and tiff files from Perkin-Elmer, Pilatus, and GE. See the [image importers code docs](https://gsas-ii.readthedocs.io/en/latest/imports.html#image-importer-routines) for a full list of the supported formats. Although many of these formats have data fields that should contain important information for the measurement (e.g. wavelength), these are rarely filled in correctly by the data acquisition software. Thus, you should have separately noted this information as it will be needed. In some cases, this information may be in a separate “metadata” file; GSAS-II will look for this and attempt to open it as well as the image file.
 
     !!! Note