Add CI for code coverage and update README (#222)

- Create coverage.yml
   - Add codecov.yml to ignore cmd and logs folders
- Update README.md
   - Add badges to show various CI status.
   - Add API Reference section and link to godoc.
   - Add Contributing section.

.github/workflows/coverage.yml

@@ -0,0 +1,34 @@
+# coverage.yml  Generate and upload Go code coverage report to Codecov.
+# https://github.com/onflow/atree/blob/main/.github/workflows/coverage.yml
+# 2021-11-26    Created. 
+
+name: coverage
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2  # allow GitHub to use latest v2.x of checkout
+        with:
+          fetch-depth: 2
+
+      - uses: actions/setup-go@v2  # allow GitHub to use latest v2.x of setup-go
+        with:
+          go-version: '1.17.x'
+
+      - name: Get dependencies
+        run: go get -v -t -d ./...
+
+      - name: Build project
+        run: go build ./...
+
+      - name: Generate coverage report
+        run: go test -timeout 60m -race -coverprofile=coverage.txt -covermode=atomic
+
+      - name: Upload coverage report to Codecov
+        uses: codecov/[email protected]  # use v2.1.0, we can manually update uploader version if needed
+        with:
+            files: ./coverage.txt
+            fail_ci_if_error: false # we can set this to true after "Could not find a repository" upload error is fixed
+            verbose: true # optional (default = false)

README.md

@@ -1,7 +1,21 @@
 # Atree
 
+[![](https://github.com/onflow/atree/workflows/ci/badge.svg)](https://github.com/onflow/atree/actions?query=workflow%3Aci)
+[![](https://github.com/onflow/atree/workflows/linters/badge.svg)](https://github.com/onflow/atree/actions?query=workflow%3Alinters)
+[![CodeQL](https://github.com/onflow/atree/actions/workflows/codeql-analysis.yml/badge.svg)](https://github.com/onflow/atree/actions/workflows/codeql-analysis.yml)
+
 Atree provides scalable arrays and scalable ordered maps.  It is used by [Cadence](https://github.com/onflow/cadence) in the [Flow](https://github.com/onflow/flow-go) blockchain.
 
+## API Reference
+
+Atree's API is [documented](https://pkg.go.dev/github.com/onflow/atree#section-documentation) with godoc at pkg.go.dev and will be updated when new versions of Atree are tagged.  Additional documentation will be added as time allows.
+
+## Contributing
+
+If you would like to contribute to Atree, have a look at the [contributing guide](https://github.com/onflow/atree/blob/main/CONTRIBUTING.md).
+
+Additionally, all non-error code paths must be covered by tests.  And pull requests should not lower the code coverage percent.
+
 ## License
 
 The Atree library is licensed under the terms of the Apache license. See [LICENSE](LICENSE) for more information.

codecov.yml

@@ -0,0 +1,3 @@
+ignore:
+  - "cmd/**/*"
+  - "logs/"