Update README.md

Author: grburgess

README.md

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 # binder_env
 The base binder environment for 3ML tutorials and binders
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+In order to not need to rebuild the binder with every change to content, this holds a complete 3ML install with all the bells and whistles. To attach a repo with tutorial content, simply add this link to the content repo:
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+`https://mybinder.org/v2/gh/threeML/binder_env/master?urlpath=git-pull?repo=<url-of-your-content-repo>`
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