Auto merge of #8715 - ehuss:contrib, r=alexcrichton

Add contributor guide.

This consolidates and extends the contributor information in a single place. This is an mdbook project, along with a CI job which will build and deploy it to GitHub Pages at <https://rust-lang.github.io/cargo/contrib/>.

You can view a rendered version here: <https://ehuss.github.io/cargo/contrib/>

I don't know if this will actually be helpful to anyone, but I figured it's worth a shot.

NOTE: The CI deploy is designed to preserve the existing gh-pages content.  However, it will **delete the history** on that branch. I think that should be fine, there doesn't seem to be too much interesting stuff there.  I do have a backup in my fork, though. Some extra scrutiny on the code might be wise.  The reason it deletes the history is because deploying mdbook on every push would balloon the repository size.

.github/ISSUE_TEMPLATE/tracking_issue.md

@@ -0,0 +1,34 @@
+---
+name: Tracking Issue
+about: A tracking issue for an accepted feature or RFC in Cargo.
+title: Tracking Issue for XXX
+labels: C-tracking-issue
+---
+<!--
+Thank you for creating a tracking issue! Tracking issues are for tracking an
+accepted feature or RFC from implementation to stabilization. Please do not
+file a tracking issue until the feature or RFC has been approved.
+-->
+
+**About tracking issues**
+
+Tracking issues are used to record the overall progress of implementation.
+They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
+A tracking issue is however *not* meant for large scale discussion, questions, or bug reports about a feature.
+Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
+
+**Summary**
+
+Original issue: #NNNN <!-- if there is a related issue that spawned this feature -->
+Implementation: #NNNN <!-- link to the PR that implemented this feature if applicable -->
+Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#my-feature
+
+<!-- Write a very brief summary of the feature here. -->
+
+**Unresolved issues**
+
+* [ ] Make a list of any known implementation or design issues.
+
+**Future extensions**
+
+<!-- An optional section where you can mention where the feature may be extended in the future, but is explicitly not intended to address. -->

.github/workflows/contrib.yml

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+name: Contrib Deploy
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+    - name: Install mdbook
+      run: |
+        mkdir mdbook
+        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.3/mdbook-v0.4.3-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
+        echo ::add-path::`pwd`/mdbook
+    - name: Deploy docs
+      run: |
+        cd src/doc/contrib
+        mdbook build
+        git worktree add gh-pages gh-pages
+        git config user.name "Deploy from CI"
+        git config user.email ""
+        cd gh-pages
+        # Delete the ref to avoid keeping history.
+        git update-ref -d refs/heads/gh-pages
+        rm -rf contrib
+        mv ../book contrib
+        git add contrib
+        git commit -m "Deploy $GITHUB_SHA to gh-pages"
+        git push --force