List required development headers in README

[phlummox]

Intended to partially fix #3269.

The explanation of what these libraries are needed for still needs to be filled in.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (39e2e3d) 85.21% compared to head (7848b3c) 85.21%.
Report is 1329 commits behind head on master.

Files	Patch %	Lines
parsers/openapi.c	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3272   +/-   ##
=======================================
  Coverage   85.21%   85.21%           
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  Files         210      210           
  Lines       49689    49689           
=======================================
  Hits        42343    42343           
  Misses       7346     7346           

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[masatake]

@phlummox, thank you.
I forgot that we have document writing about the optional packages in https://github.com/universal-ctags/ctags/blob/master/docs/autotools.rst. If you think the document is not enough, could you convert this pull request for the document?

At least, we must write about pcre2 in autotools.rst.
On Fedora, pcre2-devel is needed.

libxml2:
This allows Universal Ctags to work libxml2 based parsers like XSLT parser.

Jansson:
This allows Universal Ctags to emit tags in JSON format (--output-format=json option).
JSON parser works without Jansson.

LibYAML:
This allows Universal Ctags to work libYAML based parsers like OpenAPI parser.

pcre2:
This allows Universal Ctags to accept pcre2 regular expression when defining a parser from the command line. Currently, there is no built-i parser using pcre2.

All these are optional. Universal Ctags must be built without these libraries and their header files.

[masatake]

libseccomp is also nice to have.