Publish RFCs as mdbook to GitHub pages.

Spoke with someone at Rust Belt Rust who mentioned this idea (@mre 👋)
and decided to take a stab at it. I originally implemented this upstream
in the Rust source repository (by adding a git submodule and dynamically
generating the contents of the book there). But then then I spoke with
@steveklabnik and agreed that doing it in CI in this repository would be
a lot easier.

.gitignore

@@ -1 +1,3 @@
 *~
+book/
+src/

.travis.yml

@@ -0,0 +1,19 @@
+language: rust
+script:
+  - (cargo install mdbook --git https://github.com/azerupi/mdBook.git --force || true)
+  - mkdir src
+  - echo "[Introduction](introduction.md)\n" >> src/SUMMARY.md
+  - |
+    for f in $(ls text/* | sort)
+    do
+        echo "- [$(basename $f ".md")]($(basename $f))" >> src/SUMMARY.md
+        mv $f src
+    done
+  - cp README.md src/introduction.md
+  - mdbook build -d .
+deploy:
+  provider: pages
+  skip_cleanup: true
+  github_token: $GITHUB_TOKEN
+  on:
+    branch: master