chore: Update the .editorconfig file; add/update README files

Add various missing pieces to their right places:
 * Update .editorconfig in order to let editorconfig-checker know that
   aclocal.m4 (which is auto-generated) may contain trailing whitespace.
 * Add ci/README.md.
 * Update scripts/README.txt.

TODO:
Integrate editorconfig-checker into the linting workflow on GitHub.
(See .github/workflows/lint.yml)

ci/README.md

@@ -0,0 +1,25 @@
+Scripts for the Continuous Integration of the PNG Reference Library
+===================================================================
+
+Copyright Notice
+----------------
+
+Copyright (c) 2019-2024 Cosmin Truta.
+
+Use, modification and distribution are subject to the MIT License.
+Please see the accompanying file `LICENSE_MIT.txt` or visit
+https://opensource.org/license/mit
+
+File List
+---------
+
+    LICENSE_MIT.txt         ==>  The License file
+    README.md               ==>  This file
+    ci_lint.sh              ==>  Lint the source code
+    ci_shellify.sh          ==>  Convert select definitions to shell syntax
+    ci_verify_cmake.sh      ==>  Verify the build driven by CMakeLists.txt
+    ci_verify_configure.sh  ==>  Verify the build driven by configure
+    ci_verify_makefiles.sh  ==>  Verify the build driven by scripts/makefile.*
+    ci_verify_version.sh    ==>  Verify the consistency of version definitions
+    lib/ci.lib.sh           ==>  Shell utilities for the main ci_*.sh scripts
+    targets/*/ci_env.*.sh   ==>  Shell environments for cross-platform testing