chore: add CONTRIBUTING file

CONTRIBUTING.md

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+# Contributing
+
+Please see the [main contributing guidelines](./docs/contributing.md).
+
+There are additional docs describing [contributing documentation changes](./docs/contributing_docs.md).
+
+### GitHub Sponsorship
+
+Testcontainers is [in the GitHub Sponsors program](https://github.com/sponsors/testcontainers)!
+
+This repository is supported by our sponsors, meaning that issues are eligible to have a 'bounty' attached to them by sponsors.
+
+Please see [the bounty policy page](https://golang.testcontainers.org/bounty) if you are interested, either as a sponsor or as a contributor.

docs/bounty.md

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+# Testcontainers issue bounty policy
+
+## General
+
+We want to use issue bounties to encourage contributions in areas that are important to our sponsors, or tricky to solve.
+This includes bug fixes and new features.
+We hope that this will provide incentives to tackle issues, and gives sponsors a way to influence where development time is expended.
+We also want to reward our contributors, some of whom make huge efforts to improve Testcontainers and help their fellow developers!
+
+!!! note
+    It's early days for our use of sponsorship, so we expect to evolve this policy over time, possibly without notice. In the event of any ambiguity or dispute, the [Testcontainers org core maintainers](#organisation-core-maintainers) have the final say.
+
+    If you'd like to suggest an improvement to this policy, we'd be grateful for your input - please raise a pull request!
+
+## For Sponsors
+
+Sponsors will be able to create a number of 'bounties' per month, varying according to sponsorship tier.
+
+As a sponsor, the process for creating a bounty is as follows:
+
+* Raise an issue, or find an existing issue that describes the bug or feature.
+* Start a discussion with the [Testcontainers org core maintainers](#organisation-core-maintainers) to agree that the issue is suitable for a bounty, and how much the reward amount should be.
+* Once agreed, we will assign a label to the issue so that interested developers can find it.
+
+Sponsors can create up to 1 or 3 bounties (according to tier) _per calendar month_ - i.e. the counter resets on the 1st of each month. 
+If a sponsor does not use their full quota of bounty credits in a calendar month, they cannot be rolled over to the next month.
+
+Bounties will expire 90 days after creation - after this time, if they have not been resolved we will close them.
+
+## For Contributors
+
+As a contributor, the process for working on an issue with a bounty attached is:
+
+* Find an issue with a bounty attached to it and no assignee, clarify the requirements if necessary, and consider how you would approach working on it.
+* Start a discussion with the [Testcontainers org core maintainers](#organisation-core-maintainers) and the bounty owner. To avoid unpleasant surprises at review time, we'll try to confirm that we're happy with your proposed solution.
+* If we're happy with your proposed solution, we will assign the ticket to you.
+* Once work is complete, we will go through the PR process as usual and merge the work when finished.
+* To receive the bounty reward, [raise an invoice](https://opencollective.com/testcontainers/expenses/new) on Open Collective, following the expenses policy on that page.
+
+Note that a 20% cut of the bounty amount will normally be assigned to project maintainers for PR review work.
+We believe this reflects that PR review can often be a significant amount of work for some issues - and also gives maintainers an incentive to complete the review and unlock the bounty reward!
+Some pull requests are so well done that very little review is necessary. If that happens, the maintainers may choose not to take a cut of the bounty, and instead release the full amount to the contributor.
+
+## Organisation core maintainers
+
+The organisation core maintainers are:
+
+* Richard North (@rnorth)
+* Sergei Egorov (@bsideup)
+* Kevin Wittek (@kiview)