Clarify how to make progress on a PR (open-telemetry#745)

Author: reyang

CODEOWNERS .github/CODEOWNERS

@@ -82,3 +82,76 @@ To quickly fix typos, use
 ```bash
 make misspell-correction
 ```
+
+## Pull Requests
+
+### How to create a PR
+
+Everyone is welcome to contribute to the OpenTelemetry specification via GitHub
+pull requests (PRs).
+
+To [create a new
+PR](https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/creating-a-pull-request),
+fork the project in GitHub and clone the upstream repo:
+
+```sh
+git clone https://github.com/open-telemetry/opentelemetry-specification.git
+```
+
+Add your fork as a remote:
+
+```sh
+git remote add fork https://github.com/YOUR_GITHUB_USERNAME/opentelemetry-specification.git
+```
+
+Check out a new branch, make modifications and push the branch to your fork:
+
+```sh
+$ git checkout -b feature
+# edit files
+$ git commit
+$ git push fork feature
+```
+
+Open a pull request against the main `opentelemetry-specification` repo.
+
+If the PR is not ready for review, please mark it as
+[`draft`](https://github.blog/2019-02-14-introducing-draft-pull-requests/).
+
+Please make sure CLA is signed and CI is clear. We don't expect people to review
+and comment on a PR that doesn't have CLA signed.
+
+### How to get your PR merged
+
+A PR is considered to be **ready to merge** when:
+
+* It has received more than two approvals from the [code
+  owners](./.github/CODEOWNERS) (if approvals are from only one company, they
+  won't count).
+* There is no `request changes` from the [code owners](./.github/CODEOWNERS).
+* It has been at least two working days since the last modification (except for
+  the trivial updates, such like typo, cosmetic, rebase, etc.). This gives
+  people reasonable time to review.
+* Trivial changes (typos, cosmetic changes, CI improvements, etc.) don't have to
+  wait for two days.
+
+Any [spec
+maintainer](https://github.com/open-telemetry/community/blob/master/community-members.md#specifications-and-proto) can
+merge the PR once it is **ready to merge**.
+
+If a PR has been stuck (e.g. there are lots of debates and people couldn't agree
+on each other), the owner should try to get people aligned by:
+
+* Consolidating the perspectives and putting a summary in the PR. It is
+  recommended to add a link into the PR description, which points to a comment
+  with a summary in the PR conversation.
+* Tagging subdomain experts (by looking at the change history) in the PR asking
+  for suggestion.
+* Reaching out to more people on the [gitter
+  channel](https://gitter.im/open-telemetry/opentelemetry-specification).
+* Stepping back to see if it makes sense to narrow down the scope of the PR or
+  split it up.
+
+If none of the above worked and the PR has been stuck for more than 2 weeks, the
+owner should bring it to the [OpenTelemetry Specification SIG
+meeting](https://github.com/open-telemetry/community#cross-language-specification).

CONTRIBUTING.md

@@ -11,6 +11,7 @@
     "resource": [
       {
         "files": [
+          ".github/CODEOWNERS",
           ".markdownlint.yaml",
           ".vscode/settings.json",
           "**.png"