Clarify supported map formats for LwDITA
Clarify supported map formats for LwDITA

Fixes #232 (originally opened as https://github.com/dita-ot/dita-ot/issues/2918)

Signed-off-by: Roger Sheen <a61a1add71dadc74037ae5e44ee30713db9a7451@infotexture.net>

infotexture/docs,infotexture/docs,dita-ot/docs