ISD-2551 update README for jenkins-agent-k8s

[Thanhphan1147]

Update README to follow the template defined at : https://github.com/canonical/is-charms-template-repo

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex)

[erinecon]

Thanks for updating this README!

[github-actions]

Lint checks failed for 838d908

```

tests/integration/conftest.py:138: error: Item "Type" of "Type | None" has no attribute "units" [union-attr]
tests/integration/conftest.py:138: error: Item "None" of "Type | None" has no attribute "units" [union-attr]
tests/integration/conftest.py:191: error: Item "Type" of "Type | None" has no attribute "units" [union-attr]
tests/integration/conftest.py:191: error: Item "None" of "Type | None" has no attribute "units" [union-attr]
Found 4 errors in 1 file (checked 20 source files)

[github-actions]

Test coverage for 838d908

Name              Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------
src/agent.py         60      0     12      0   100%
src/charm.py         57      0     10      0   100%
src/metadata.py       8      0      0      0   100%
src/pebble.py        23      0      0      0   100%
src/server.py        50      0     12      0   100%
src/state.py         66      1     10      1    97%   99
-------------------------------------------------------------
TOTAL               264      1     44      1    99%

Static code analysis report

Run started:2024-12-20 15:38:37.770850

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1669
  Total lines skipped (#nosec): 1
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):

[erinecon]

One more nitpick and then should be good to go!

[erinecon]

Suggested change

	- [Matrix](https://matrix.to/#/#charmhub-charmdev:ubuntu.com)
	* [Matrix](https://matrix.to/#/#charmhub-charmdev:ubuntu.com)

The rendering looks a little strange with the -