Scaffold: Add new expense policy

EXPENSE_POLICY.md

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+# Expense Policy
+
+## Funding can be requested for significant changes made by Core Members.
+* Discuss the changes in the private gulp team forum.
+* Include a cost estimation with either a fixed price or hours + rate (suggested $50 per hour).
+* Notify the team before you exceed an estimate.
+
+## Bug bounties may be assigned at the Core Members’ discretion to issues of significant importance - usually issues outstanding for at least 6 months.
+* Issues with bug bounties will be labeled “Bug Bounty: $x”.
+* In order to claim a bug bounty, create a Pull Request that fixes an issue with a “Bug Bounty” label.
+* The Pull Request must be reviewed and merged by a Core Member. If competing submissions exist, the best solution will be chosen by a Core Member. All else equal, the first submission will be chosen.
+* Once your Pull Request is merged, you can submit an expense to our [Open Collective](https://opencollective.com/gulpjs/expenses/new) which includes the link to your submission in the description (e.g. $100 bug bounty claim for https://github.com/gulpjs/gulp/pull/2226). You will also need to provide an invoice, see the [Open Collective Expense FAQ](https://opencollective.com/faq/expenses) for more details and to get a Google Docs template that you can use.
+* Then, add a comment on your Pull Request, noting that you’ve claimed the money, with a link to your Open Collective expense. This is to ensure the same person who fixed the issue is claiming the money.
+* Your expense will be validated by a Core Member and then your payment will be dispersed by Open Collective the following Friday.
+
+## If you're doing other good things for gulp that end up costing you real money, feel free to reach out and we can discuss helping with those expenses!