backend: add documentation on how to run the payment api locally

backend/doc/payment.md

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+# Open collective payment
+
+The payment method we use for jobsika is based off of the opencollective [GraphQL API](https://graphql-docs-v2.opencollective.com/welcome).
+We are using opencollective because the money that we receive lands directly on our opencollective account.
+Which makes it easier to access that money and spend it in the open.
+
+### How to develop ?
+
+You are a developer and you would like to test or modify our payment solution. You are at the right the place,
+this document aims to provide you enough information to get you started.
+
+#### Prerequisites
+
+In order to contribute to the payment solution you will need to have an account to opencollective, and have admin
+access to an organisation.
+
+You can request admin access to [openbilling](https://opencollective.com/openbilling), [osscameroon](https://opencollective.com/osscameroon) or create your own organisation.
+
+
+Once you have admin access to an organisation, you need to generate an API Key. To do so, go to your personal settings on opencollective select the `For developers` section and create a new personal token.
+Make sure you to set the `Account` and `Webhooks` scopes for that personal token.
+
+**For developers tab**
+
+![For developers](./res/opencollective-for-developers.png)
+
+**Create a new token**
+
+![Create a new token](./res/create-a-new-token.png)
+
+
+**Copy your personal token**
+
+![Copy your personal token](./res/copy-your-personal-token.png)
+
+
+#### Setup
+
+Once you have your personal token, clone the [jobsika](https://github.com/osscameroon/jobsika) repository and head to the `./backend` folder.
+Copy the `.opencollective-env-example` and rename it to `.opencollective-env`. Then replace the `OPENCOLLECTIVE_API_KEY` with your personal token.
+and the `OPENCOLLECTIVE_ORG_SLUG` with the slug of the organisation you have admin access to.
+
+Now you should be able to run the backend and make payment requests.
+
+Run the api using `make start-api`
+
+Then run the command `curl -vsS -X POST -H 'Content-Type: application/json' -d '{"email":"[email protected]", "tier": "je suis con", "job_offer_id": "1"}' http://localhost:7000/pay | jq`
+
+You should have a similar output to:
+
+```
+{
+  "tier_url": "https://opencollective.com/osscameroon/contribute/jobsika-joboffer-56908"
+}
+```
+
+The `tier_url` is the link to the newly created payment tier.
+
+
+Before you can proceed to the next step you must setup a `Webhook`. Go to the organisation profile and select the `Webhooks` tab.
+Then click on `Add a new webhook` and fill the form as shown bellow.
+
+You will need to expose your local api to the internet. You can use [ngrok](https://ngrok.com/) to do so.
+
+Once you have generated a public url, you can set the URL field to `https://<whatever-your-ngrok-domain-is>/open-collective-webhook`.
+The `/open-collective-webhook` is the endpoint that the api exposes to receive webhooks from opencollective.
+
+**Webhook form**
+[![Webhook form](./res/webhook-form.png)](./res/webhook-form.png)
+
+
+Now you can go back to the `tier_url` your received from the previous payment request and click on the `Pay now` button.