Updated README.md

README.md

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 Add as a first buildpack in the chain. Set `PROJECT_PATH` environment variable to point to project root. It will be promoted to slug's root, everything else will be erased. Following buildpack (e.g. nodejs) will finish slug compilation.
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+**Disclaimer:** I may change the code without notice, so always pin to specific github version. Provided as is.
+
+# How to use:
+1. `heroku buildpacks:clear` if necessary
+2. `heroku buildpacks:set https://github.com/timanovsky/subdir-heroku-buildpack`
+3. `heroku buildpacks:add heroku/nodejs` or whatever buildpack you need for your application
+4. `heroku config:set PROJECT_PATH=projects/nodejs/frontend` pointing to what you want to be a project root.
+5. Deploy your project to Heroku.
+
+# How it works
+The buildpack takes subdirectory you configured, erases everything else, and copies that subdirectory to project root. Then normal Heroku slug compilation proceeds.