Update contributing guide with new label policy

docs/CONTRIBUTING.adoc

@@ -28,7 +28,8 @@ Merging pull requests once CI is successful:
 . Once a PR is ready for review please add the https://github.com/paritytech/substrate/pulls?q=is%3Apr+is%3Aopen+label%3AA0-pleasereview[`A0-pleasereview`] label. Generally PRs should sit with this label for 48 hours in order to garner feedback. It may be merged before if all relevant parties had a look at it.
 . If the first review is not an approval, swap `A0-pleasereview` to any label `[A3, A7]` to indicate that the PR has received some feedback, but needs further work. For example. https://github.com/paritytech/substrate/labels/A3-inprogress[`A3-inprogress`] is a general indicator that the PR is work in progress and https://github.com/paritytech/substrate/labels/A4-gotissues[`A4-gotissues`] means that it has significant problems that need fixing. Once the work is done, change the label back to `A0-pleasereview`. You might end up swapping a few times back and forth to climb up the A label group. Once a PR is https://github.com/paritytech/substrate/labels/A8-mergeoncegreen[`A8-mergeoncegreen`], it is ready to merge. 
 . PRs must be tagged with respect to _release notes_ with https://github.com/paritytech/substrate/labels/B0-silent[`B0-silent`] and `B1-..`. The former indicates that no changes should be mentioned in any release notes. The latter indicates that the changes should be reported in the corresponding release note
-. PRs that break the external API must be tagged with https://github.com/paritytech/substrate/labels/B2-breaksapi[`B2-breaksapi`], when it changes the FRAME or consensus of running system with https://github.com/paritytech/substrate/labels/B3-breaksconsensus[`B3-breaksconsensus`]
+. PRs that break the external API must be tagged with https://github.com/paritytech/substrate/labels/B2-breaksapi[`B2-breaksapi`], when it changes the FRAME or consensus of running system with https://github.com/paritytech/substrate/labels/B3-breaksconsensus[`B3-breaksconsensus`].
+. PRs should be labeled with their release importance via the `C1-C9` labels as well as their project/scope via the `M*` labels.
 . No PR should be merged until all reviews' comments are addressed.
 
 *Reviewing pull requests*:

docs/PULL_REQUEST_TEMPLATE.md

@@ -6,11 +6,11 @@ Before you submitting, please check that:
   - What does it do?
   - What important points reviewers should know?
   - Is there something left for follow-up PRs?
-- [ ] You labeled the PR with appropriate labels if you have permissions to do so.
+- [ ] You labeled the PR with appropriate labels (`A*` for review state, `B*` for changelog, `C*` for release note, `M*` for project/scope) if you have permissions to do so.
 - [ ] You mentioned a related issue if this PR related to it, e.g. `Fixes #228` or `Related #1337`.
 - [ ] You asked any particular reviewers to review. If you aren't sure, start with GH suggestions.
-- [ ] Your PR adheres [the style guide](https://wiki.parity.io/Substrate-Style-Guide)
-  - In particular, mind the maximal line length.
+- [ ] Your PR adheres to [the style guide](https://wiki.parity.io/Substrate-Style-Guide)
+  - In particular, mind the maximal line length of 100 (120 in exceptional circumstances).
   - There is no commented code checked in unless necessary.
   - Any panickers have a proof or removed.
 - [ ] You bumped the runtime version if there are breaking changes in the **runtime**.