doc: fix readme badges

github/markup#1801 (comment)
https://docutils.sourceforge.io/docs/ref/rst/directives.html#image

Signed-off-by: Stephen Whitlock <stephen@jshwisolutions.com>

README.rst

@@ -7,49 +7,51 @@
 
 |
 
-.. image:: https://img.shields.io/badge/License-MIT-yellow.svg
+|License| |PyPI| |CI| |CodeQL| |pre-commit.ci status| |codecov.io| |readthedocs.org| |python3.8| |Black| |isort| |docformatter| |pylint| |Security Status| |Known Vulnerabilities| |docsig|
+
+.. |License| image:: https://img.shields.io/badge/License-MIT-yellow.svg
    :target: https://opensource.org/licenses/MIT
    :alt: License
-.. image:: https://img.shields.io/pypi/v/docsig
+.. |PyPI| image:: https://img.shields.io/pypi/v/docsig
    :target: https://pypi.org/project/docsig/
    :alt: PyPI
-.. image:: https://github.com/jshwi/docsig/actions/workflows/build.yaml/badge.svg
+.. |CI| image:: https://github.com/jshwi/docsig/actions/workflows/build.yaml/badge.svg
    :target: https://github.com/jshwi/docsig/actions/workflows/build.yaml
    :alt: CI
-.. image:: https://github.com/jshwi/docsig/actions/workflows/codeql-analysis.yml/badge.svg
+.. |CodeQL| image:: https://github.com/jshwi/docsig/actions/workflows/codeql-analysis.yml/badge.svg
    :target: https://github.com/jshwi/docsig/actions/workflows/codeql-analysis.yml
    :alt: CodeQL
-.. image:: https://results.pre-commit.ci/badge/github/jshwi/docsig/master.svg
+.. |pre-commit.ci status| image:: https://results.pre-commit.ci/badge/github/jshwi/docsig/master.svg
    :target: https://results.pre-commit.ci/latest/github/jshwi/docsig/master
    :alt: pre-commit.ci status
-.. image:: https://codecov.io/gh/jshwi/docsig/branch/master/graph/badge.svg
+.. |codecov.io| image:: https://codecov.io/gh/jshwi/docsig/branch/master/graph/badge.svg
    :target: https://codecov.io/gh/jshwi/docsig
    :alt: codecov.io
-.. image:: https://readthedocs.org/projects/docsig/badge/?version=latest
+.. |readthedocs.org| image:: https://readthedocs.org/projects/docsig/badge/?version=latest
    :target: https://docsig.readthedocs.io/en/latest/?badge=latest
    :alt: readthedocs.org
-.. image:: https://img.shields.io/badge/python-3.8-blue.svg
+.. |python3.8| image:: https://img.shields.io/badge/python-3.8-blue.svg
    :target: https://www.python.org/downloads/release/python-380
    :alt: python3.8
-.. image:: https://img.shields.io/badge/code%20style-black-000000.svg
+.. |Black| image:: https://img.shields.io/badge/code%20style-black-000000.svg
    :target: https://github.com/psf/black
    :alt: Black
-.. image:: https://img.shields.io/badge/%20imports-isort-%231674b1?style=flat&labelColor=ef8336
+.. |isort| image:: https://img.shields.io/badge/%20imports-isort-%231674b1?style=flat&labelColor=ef8336
    :target: https://pycqa.github.io/isort/
    :alt: isort
-.. image:: https://img.shields.io/badge/%20formatter-docformatter-fedcba.svg
+.. |docformatter| image:: https://img.shields.io/badge/%20formatter-docformatter-fedcba.svg
    :target: https://github.com/PyCQA/docformatter
    :alt: docformatter
-.. image:: https://img.shields.io/badge/linting-pylint-yellowgreen
+.. |pylint| image:: https://img.shields.io/badge/linting-pylint-yellowgreen
    :target: https://github.com/PyCQA/pylint
    :alt: pylint
-.. image:: https://img.shields.io/badge/security-bandit-yellow.svg
+.. |Security Status| image:: https://img.shields.io/badge/security-bandit-yellow.svg
    :target: https://github.com/PyCQA/bandit
    :alt: Security Status
-.. image:: https://snyk.io/test/github/jshwi/docsig/badge.svg
+.. |Known Vulnerabilities| image:: https://snyk.io/test/github/jshwi/docsig/badge.svg
    :target: https://snyk.io/test/github/jshwi/docsig/badge.svg
    :alt: Known Vulnerabilities
-.. image:: https://snyk.io/advisor/python/docsig/badge.svg
+.. |docsig| image:: https://snyk.io/advisor/python/docsig/badge.svg
    :target: https://snyk.io/advisor/python/docsig
    :alt: docsig