doc: tidy the text files

* etc/README: Rename/reformat as...
* etc/README.md: this.
And ship it.

etc/README.md

@@ -0,0 +1,36 @@
+# bench.pl
+A primitive Perl script to run benches.  Currently its only bench is the
+usual calculator: it has a lightweight processing part (but not null), so it
+is really emphasizing the cost of the whole parsing (including scanning and
+I/O).
+
+Set the envvars `BISON` and `CC` to the programs and flags you want to use.
+They default to `bison` and `gcc`.  To run a non-installed bison from a
+tarball built in _build, I suggest the following.
+
+    BISON=_build/tests/bison CC='gcc -O2' ./bench.pl
+
+(The `_build/tests/bison` wrapper makes sure that `_build/src/bison` will
+use the tarballs' skeletons, not those already installed as a
+straightforward use of `_build/src/bison` would.)
+
+<!--
+
+Local Variables:
+mode: markdown
+fill-column: 76
+ispell-dictionary: "american"
+End:
+
+Copyright (C) 2006, 2009-2015, 2018-2020 Free Software Foundation, Inc.
+
+This file is part of Bison, the GNU Compiler Compiler.
+
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.3 or
+any later version published by the Free Software Foundation; with no
+Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
+Texts.  A copy of the license is included in the "GNU Free
+Documentation License" file as part of this distribution.
+
+-->