Add and document bounty system

- Add mention in the README
- add new workflow label.yml, which uses github actions to post the
rules of the bounty as a comment in an issue labelled 'bounty'
- add docs/bounties.md where the bounty system is documented

.github/workflows/label.yml

@@ -0,0 +1,19 @@
+# This file is maintained in https://github.com/WeblateOrg/meta/
+name: Comment to label
+
+on:
+  issues:
+    types: [labeled]
+
+jobs:
+  issueLabeled:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Bounty explanation
+        uses: peter-evans/create-or-update-comment@v1
+        if: github.event.label.name == '💰bounty'
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          issue-number: ${{ github.event.issue.number }}
+          body: |
+            There is a bounty on this issue, the amount is in the title. The bounty will be awarded to the first person(s) who resolves this issue. Read the full conditions in the ['bounties.md' file](https://github.com/erciccione/haveno/blob/master/docs/bounties.md). If you are starting to work on this issue, please write a comment here, so that we can assign the issue to you and avoid duplicated work.