Added CHANDELOG.md and remove table of contents from README.markdown

Testing done

### Submitter checklist
- [ ] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [ ] Ensure that the pull request title represents the desired changelog entry
- [X ] Please describe what you did
- [ X] Link to relevant issues in GitHub or Jira
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

jenkinsci / zulip-plugin

Added CHANDELOG.md and remove table of contents from README.markdown #45

Testing done