create guide for txt tag rendition

[njr2128]

Based on table below, a user guide should be created for all txt files.
Put into a READ.ME in every directory of txt files (e.g., ms-txt, entry/txt, allFolios/txt)

Element name	Rendition in DCE	Rendition in txt	Entry-metadata
Addition	<add>TEXT</add>	TEXT	plaintext
Correction	<corr>TEXT</corr>	[TEXT]	plaintext
Deletion	<del>TEXT</del>	<-TEXT->	Excluded from entry-metadata headings
Emphasis	<emph>TEXT</emph>	TEXT	plaintext
Expansion	<exp>TEXT</exp>	{TEXT}	plaintext
Gap	<gap/>	omitted	omitted
Illegible	<ill/>	[illegible]	omitted
Superscript	<superscript>TEXT</superscript>	omitted	omitted
Supplied text	<sup>TEXT</sup>	[TEXT]	plaintext
Underline	<underline>TEXT</underline>	TEXT	plaintext