Added minimum version info to setup.cfg metadata

This also documents that the long_description_content_type keyword was
added in version 38.6.0.
Co-authored-by: Gene Matusovsky <gene_matusovsky@s5a.com>

docs/setuptools.txt

@@ -2434,9 +2434,9 @@ Metadata
     The aliases given below are supported for compatibility reasons,
     but their use is not advised.
 
-==============================  =================  =====
-Key                             Aliases            Type
-==============================  =================  =====
+==============================  =================  =================  ===============
+Key                             Aliases            Type               Minumum Version
+==============================  =================  =================  ===============
 name                                               str
 version                                            attr:, file:, str
 url                             home-page          str
@@ -2450,13 +2450,13 @@ classifiers                     classifier         file:, list-comma
 license                                            file:, str
 description                     summary            file:, str
 long_description                long-description   file:, str
-long_description_content_type                      str
+long_description_content_type                      str                38.6.0
 keywords                                           list-comma
 platforms                       platform           list-comma
 provides                                           list-comma
 requires                                           list-comma
 obsoletes                                          list-comma
-==============================  =================  =====
+==============================  =================  =================  ===============
 
 .. note::
     A version loaded using the ``file:`` directive must comply with PEP 440.