doc: scripts to run embedmd on markdown files

This will help keep READMEs in sync with code.

Also adds a README for the add example.

Updates #14

script/bootstrap

@@ -8,3 +8,6 @@ go get -u \
 # Install golangci-lint
 golangci_lint_version='v1.12.3'
 curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $GOPATH/bin ${golangci_lint_version}
+
+# embedmd required for documentation generation
+go get -u github.com/campoy/embedmd