Update contributing spec with list of spec

CONTRIBUTING.md

@@ -20,15 +20,28 @@ OpenMapTiles is an open source project and we love to receive contributions from
 
 # Code review process
 
-We all make mistakes and bad coding decisions. So apart from the obvious fixes, all changes must be reviewed by another member of the project. This also helps with the [bus factor](https://en.wikipedia.org/wiki/Bus_factor) -- there should always be another person on the team who knows why a change was made.
+We all make mistakes and bad coding decisions. So apart from the obvious fixes, all changes must be reviewed by another 2 members of the project. This also helps with the [bus factor](https://en.wikipedia.org/wiki/Bus_factor) -- there should always be other people in the team who know why a change was made.
 
-For any non-trivial changes, all pull requests must be approved by at least one member of the OpenMapTiles team. Afterwards you can merge the PR if you have the rights, or another person must do it for you.
+For any non-trivial changes, all pull requests must be approved by at least three members of the OpenMapTiles team. Afterwards you can merge the PR if you have rights, or another person must do it for you.
 
-As a rule of thumb, changes are obvious fixes if they do not introduce any new functionality or creative thinking. As long as the change does not affect functionality, some likely examples include the following:
-* Spelling / grammar fixes
-* Typo correction, white space and formatting changes
-* Comment clean up
-* Changes to ‘metadata’ files like .gitignore, build scripts, etc.
-* Moving source files from one directory to another
+Your pull request must:
 
-_This policy was adapted from the CC0-licensed [nayafia/contributing-template](https://github.com/nayafia/contributing-template)_
+ * Address a single issue or add a single item of functionality.
+ * Contain a clean history of small, incremental, logically separate commits,
+   with no merge commits.
+ * Use clear commit messages.
+ * Be possible to merge automatically.
+
+When you modify rules of importing data in `mapping.yaml` or `*.sql`, please update also:
+
+1. field description in `[layer].yaml`
+2. comments starting with `#etldoc`
+3. if needed, generate new `mapping_diagram.png` or `etl_diagram.png` using commands below:
+```
+make mapping-graph-[layer]
+make etl-graph-[layer]
+```
+4. update layer description on https://openmaptiles.org/schema/ (https://github.com/openmaptiles/www.openmaptiles.org/tree/master/layers)
+5. check if OMT styles are affected by the PR and if there is a need for style updates
+
+When you are making PR that adds new spatial features to OpenMapTiles schema, please make also PR for at least one of our GL styles to show it on the map. Visual check is crucial.