docs: add funding and stipend document

FUNDING.md

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+# Funding
+
+## Raising Funds
+
+Vitest is an MIT-licensed open source project and completely free to use. However, the amount of effort needed to maintain and develop new features for Vitest and its ecosystem is not sustainable without proper financial backing. We need your help to achieve this.
+
+Learn more about sponsorship on our [Open Collective](https://opencollective.com/vitest).
+
+### Why Open Collective?
+
+- **Full Transparency.** Everyone gets to see where money is coming from and where it's going.
+- **Individual and Corporate Sponsors.** Open Collective makes it easy for both individuals and companies to sponsor open source projects.
+- **Potential Tax Benefits.** Because funds are paid to the Open Source Collective, a 501(c)(6) organization in the U.S., there may be tax benefits for some donors (please check with your accountant).
+- **Automatic Invoicing.** For corporate sponsors, Open Collective automatically generates and sends invoices for tracking purposes.
+- **Open Participation.** Anyone can request reimbursement for funds spent helping the Vitest project & ecosystem and Vitest can pay out to anyone.
+
+_List borrowed from [ESLint: "Funding ESLint's Future."](https://eslint.org/blog/2019/02/funding-eslint-future)_
+
+## Distributing Funds
+
+100% of money raised is invested back into the Vitest ecosystem and dependencies. For example:
+
+- **Improve codebase.**
+- **Improve documentation.**
+- **Improve translations.**
+- **Improve website.**
+- **Support upstream dependencies.**
+- **Support important ecosystem plugins.**
+- And more that will benefit Vitest in some way.
+
+See all past expenses on our [Open Collective](https://opencollective.com/vitest/expenses).
+
+### Core Team Member Stipend
+
+A stipend of up to $3000 USD per calendar month is available to all core team members. Any time spent improving or supporting the project in some way counts towards this stipend: writing code, writing documentation, triaging issues, offering support in our Discord server, attending meetings (and meetups) on behalf of Vitest, contributing to our upstream dependencies, and so on, unless you are already being paid for this work under another contract or agreement. For example, if you are paid by your employer or otherwise contracted to contribute to Vitest, you should not also claim a stipend for those hours.
+
+Right now, the stipend is calculated per hour at $50 USD per hour contributed, with a maximum of $3000 USD each month. These numbers may shift as our funding and team size change over time.
+
+To claim, file an expense on our [Open Collective](https://opencollective.com/vitest) with a link to this section as _Additional Information_: https://github.com/vitest-dev/.github/blob/main/FUNDING.md#core-team-member-stipend. You may file in any currency Open Collective supports, as long as the converted amount does not exceed $3000 USD. The number of hours do not need to be described as we trust maintainers to report the amount in good faith.
+
+## Attribution
+
+This document is adapted from [Vite's Funding document](https://github.com/vitejs/.github/blob/main/FUNDING.md).