Create join.md
Create join.md

create initial page

Former-commit-id: 9e0432cb8281a5325a90851b7e68bf610f95e97e [formerly 589a29efd6603ce967e5e3233b54efa072d0f560]
Former-commit-id: 128f092be515288268a8dc31eca4373b28fc9e9e
LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io