The ReadmeName directive sets the name of the file that will be appended to the end of the index listing. Filename is the name of the file to include, and is taken to be relative to the location being indexed.
The filename argument is treated as a stub filename in Apache 1.3.6 and earlier, and as a relative URI in later versions. Details of how it is handled may be found under the description of the HeaderName directive, which uses the same mechanism and changed at the same time as ReadmeName.
See also HeaderName.