Add FTP prerequisites to ReadMe

[lauren-li]

Proposed changes

Resolves #1242 ([zFTP] Add pre-requisite in readMe file). No code changes.

Release Notes

Milestone: 1.13.1

Changelog: Add z/OS FTP Plug-in for Zowe CLI as a prerequisite to Zowe Explorer Extension for FTP.

Types of changes

What types of changes does your code introduce to Zowe Explorer?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Updates to Documentation or Tests (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This checklist will be used as reference for both the contributor and the reviewer

• I have read the CONTRIBUTOR GUIDANCE wiki
• PR title follows Conventional Commits Guidelines
• PR Description is included
• gif or screenshot is included if visual changes are made
• yarn workspace vscode-extension-for-zowe vscode:prepublish has been executed
• All checks have passed (DCO, Jenkins and Code Coverage)
• I have added unit test and it is passing
• I have added integration test and it is passing
• There is coverage for the code that I have added
• I have tested it manually and there are no regressions found
• I have added necessary documentation (if appropriate)
• Any PR dependencies have been merged and published (if appropriate)

[IgorCATech]

Look good, @lauren-li I've just left some minor comments. Thanks for adding the prerequisites to this document :)

[lauren-li]

Thanks @IgorCATech for the great suggestions! I have made the proposed changes.

[katelynienaber]

Small change...

[katelynienaber]

LGTM :)