Add contributor guide.

Author: ehuss

.github/ISSUE_TEMPLATE/tracking_issue.md

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+---
+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