add deploy script

assets/CNAME

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+exercise.rs

deploy

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+## this script deploys the static website of course.rs to github pages
+
+## build static website for book
+mdbook build
+## copy CNAME info to book dir
+cp ./assets/CNAME ./book/
+cp ./assets/*.html ./book/
+cp ./assets/sitemap.xml ./book/
+
+## init git repo
+cd book
+git init
+git config user.name "sunface"
+git config user.email "cto@188.com"
+git add .
+git commit -m 'deploy'
+git branch -M gh-pages
+git remote add origin https://github.com/course-rs/rust-exercise
+
+## push to github pages
+git push -u -f origin gh-pages