[User story] Add content to README.md

## Description
As a potential ContribMatch API developer or ContribMatch-API-based app developer, I would like to know what the project is about, so that I can determine if it fits my needs and/or whether I would like to contribute to it.

## Acceptance criteria
- Upon opening the repo on GitHub, developers **must** be able to see a description of the following: what ContribMatch is, what it tries to solve, and how ContribMatch-server fits in the picture.
- Upon opening the repo on GitHub, developers **should** be able to see a description including the following: The project's code of conduct, the name of the license of the repository with a link to it, and contributing guidelines or a link to them.
- Upon opening the repo on GitHub, developers **could** be able to see a description including the following: Security policy, or a link to it.

## Engineering tasks
- Set up the README.md and associated files.

## Time estimates
- **Story points:** 2.
- **Deadline:** 2025-05-23.