Introduce reviewing guide

Introduce `REVIEWING.md` file.

Signed-off-by: Razvan Deaconescu <[email protected]>

REVIEWING.md

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+# Reviewing Contributions
+
+These are recommendations when reviewing contributions to the Open Education Hub Methodology repository.
+Contributions will mostly take the form of GitHub pull requests (PRs);
+issues and discussions are expected as well.
+
+When reviewing contributions, please be civil and welcoming towards contributors.
+Contributions may not be in the best shape initially, but we appreciate time and effort spent in doing this.
+
+Make sure contributions follow the [contribution guide](CONTRIBUTING.md): proper formatting, proper wording, proper styling.
+See if the contributions fit into the scope of the repository and solve a problem or add value to it.
+
+If a contribution is OK, please approve it.
+In case of a reviewed and approved PR, the assignee will then `Rebase and merge` the PR.
+
+If a contribution needs work, please make the appropriate comments.
+For a PR use discussion comments, or comments on the code / text, or directly [suggest changes](https://thenextweb.com/news/github-launches-suggested-changes-to-make-it-easier-to-collaborate-within-pull-requests).