Update FMTM Field Mapping Guidelines.md

[manjitapandey]

Added FMTM guidelines for field mappers

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• 🤖 Build or CI
• ❓ Other (please specify)

Related Issue

Example: Related #921

Describe this PR

This PR adds the FMTM guidelines for field mappers including constraints of FMTM for field mappers.

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Alternative Approaches Considered

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Review Guide

Notes for the reviewer. How to test this change?

Checklist before requesting a review

• 📖 Read the FMTM Contributing Guide: https://github.com/hotosm/fmtm/blob/main/CONTRIBUTING.md
• 📖 Read the HOT Code of Conduct: https://docs.hotosm.org/code-of-conduct
• 👷‍♀️ Create small PRs. In most cases, this will be possible.
• ✅ Provide tests for your changes.
• 📝 Use descriptive commit messages.
• 📗 Update any related documentation and include any relevant screenshots.

[optional] What gif best describes this PR or how it makes you feel?

[spwoodcock]

I merged some of the docs together a bit to simplify things.
(there was a little bit of duplicated info as a result that I removed)
Hope that's ok! 😄

Ideally:

• One mapper guide
• One manager guide