Updating the description about installing in README and/or autotools.rst

[masatake]

This tracks the status of some issues and pull-requests about the topic

• write about we support only GNU make explicitly (Limit the support for building with bsd make #3353, buildsys: rewite man/Makefile.am #3349)
• Add autotools install to README #3302
• Build instructions seem incomplete - required/useful development headers not mentioned #3269
• List required development headers in README #3272

[leleliu008]

I think we can add following in our README.md

dependencies

tool	required?	purpose	note
automake	required	for generating Makefile.in from Makefile.am
autoconf	required	for generating configure script
pkg-config>=0.18	required	for finding libraries	pkgconf can be installed as a alternative.
gmake	required	for doing jobs that read from Makefile	bmake is not supported.
python>=3.5	optional	for providing a runtime for docutils
docutils	optional	for generating manpages	if can't found, manpages will not be installed.

library	required?	purpose	note
libiconv	optional	text coding convert support.	this library have been already included in some platform.
pcre2	optional	regex support	default regex library is gnulib-regex which is included in our source code. if you want to use pcre2, you should know the differences between POSIX Regular Expresion syntax and Perl5 Regular Expresion syntax.
libxml2	optional	XML support
libyaml	optional	YAML support
jansson	optional	JSON support
libseccomp	optional	sandbox support	only available on GNU/Linux

[leleliu008]

here libtool is motioned, actually, libtool is not used in our project.

[leleliu008]

maybe change from on GNU/Linux, OSX, or Windows 10 WSL, to on Unix-like sysytem or environment is better.

[leleliu008]

supporting systems and subsystems

system	subsystem	testing in our CI	testing config
Ubuntu		18.04 , 20.04	link
AlplineLinux		3.12 , 3.13 , 3.14 , 3.15	link
FreeBSD		13.0	link
OpenBSD		6.9	link
NetBSD		9.2	link
macOS		10.15 , 11.0	link
Windows	Cygwin	latest version	link
Windows	MSYS2	latest version	link
Windows	WSL	no testing
Windows			link
Android	Termux		link

Note: All GNU/Linux systems are supported, here we list only that are testing in our CI.

[masatake]

@leleliu008, thank you for updating.