docs: Added CONTRIBUTING.md (#487)

CONTRIBUTING.md

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+# Contributing to the MDIP Keychain open source project
+
+1. [Create an issue](https://github.com/KeychainMDIP/kc/issues) first
+    - Requirements and acceptance criteria should be discussed in the issue
+2. Create a development branch from the issue
+    - Branch name should start with issue number
+    - Github provides a link in the issue to [create the branch](https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/creating-a-branch-for-an-issue)
+3. Create a PR from the development branch
+    - All PRs must be linked to an issue
+4. If the PR contains code changes then do a smoke test before merge
+    - Use `./start-node` to build and run the containers
+    - Make sure the node syncs OK
+    - Check the service logs for any errors
+    - An easy check is to run `./kc perf-test` (to create 100 local ephemeral credentials) followed by `./admin verify-db` (to garbage-collect the test credentials)
+5. Merge the PR with squashed commits
+6. Use [conventional commit](https://www.conventionalcommits.org/en/v1.0.0/) prefixes for the merge commit message