[BUG] fileExtensions are not documented in README.md

[marcindulak]

Describe the bug
fileExtensions are not documented in README.md

To Reproduce
Steps to reproduce the behavior:

git clone https://github.com/UmbrellaDocs/linkspector
cd linkspector
git checkout cd42f35f5266e15743d0bd89e2b283e6cbe45f14
export BUILDKIT_PROGRESS=plain
docker build --build-arg LINKSPECTOR_PACKAGE= -t umbrelladocs/linkspector .

git show --oneline --no-abbrev --shortstat
cd42f35f5266e15743d0bd89e2b283e6cbe45f14 (HEAD, origin/asciidoc-updates) feat: Implement comprehensive AsciiDoc reference validation

docker run --rm -it -v $PWD:/app --name linkspector umbrelladocs/linkspector bash

Inside of the container:

cd /home/node/.npm-global/lib/node_modules/@umbrelladocs/linkspector/fixtures/asciidoc

echo "files:" > example.yml
echo "  - comments/test-comment-handling.adoc" >> example.yml

linkspector check --config example.yml
⠋ ℹ️ The file "comments/test-comment-handling.adoc" specified in the config does not have the correct extension. Use "fileExtensions" to configure the extensions.
✨ Success: All hyperlinks in the specified files are valid.

echo "fileExtensions:" >> example.yml
echo "  - adoc" >> example.yml

linkspector check --config example.yml
comments/test-comment-handling.adoc:28:1: 🚫 https://broken-url-for-comment-test.com Status:null net::ERR_NAME_NOT_RESOLVED at https://broken-url-for-comment-test.com
💥 Error: Some hyperlinks in the specified files are invalid.

Expected behavior
The fileExtensions option is documented in README.md

Screenshots

Additional context