Document "calt" requirement and for Code39/128 "space" workaround (Microsoft Word etc.)

[graphicore]

For all Libre Barcode fonts in MS-Word ideally the calt/"Contextual Alternates" feature must be activated (as shown in https://github.com/graphicore/librebarcode/issues/28#issuecomment-745736391). EAN-13 has documentation for workarounds (fallback/compatible).

If for Code39/128 calt is not available or not activated, the "space" character can be substituted by Â LATIN CAPITAL LETTER A WITH CIRCUMFLEX

This was necessary to fix #25 where the space glyph having "ink" caused trouble in Google Chrome/Chromium in Google Docs.

Should be documented in https://graphicore.github.io/librebarcode/documentation/code39 and https://graphicore.github.io/librebarcode/documentation/code128.html