Manual: automated link checking using mdbook-linkcheck (#61)

* travis.yml: add before_script instruction for mdbook and mdbook-linkcheck

* book.toml: add mdbook-linkcheck config options

* Update book.toml

.travis.yml

@@ -5,6 +5,8 @@ env:
 
 before_script:
   - rustup component add rustfmt
+  - if [[ -n "$BUILD_DOCS" ]]; then (cargo install mdbook --force || true); fi
+  - if [[ -n "$BUILD_DOCS" ]]; then (cargo install mdbook-linkcheck --force || true); fi
 
 matrix:
   fast_finish: true

docs/book.toml

@@ -11,3 +11,11 @@ create-missing = false
 [output.html]
 git-repository-url = "https://github.com/async-rs/async-std"
 git-repository-icon = "fa-github"
+
+[output.linkcheck]
+# Should we check links on the internet? Enabling this option adds a
+# non-negligible performance impact
+follow-web-links = false
+# Are we allowed to link to files outside of the book's root directory? This
+# may help prevent linking to sensitive files (e.g. "../../../../etc/shadow")
+traverse-parent-directories = false