docs: organize docs

break down readme into multiple files
organize in docs directory
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docs/custom-apps-for-production.md

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+# Custom apps
+
+To add your own Frappe/ERPNext apps to the image, we'll need to create a custom image with the help of a unique wrapper script
+
+> For the sake of simplicity, in this example, we'll be using a place holder called `[custom]`, and we'll be building off the edge image.
+
+Create two directories called `[custom]-worker` and `[custom]-nginx` in the `build` directory.
+
+```shell
+cd frappe_docker
+mkdir ./build/[custom]-worker ./build/[custom]-nginx
+```
+
+Create a `Dockerfile` in `./build/[custom]-worker` with the following content:
+
+```Dockerfile
+FROM frappe/erpnext-worker:edge
+
+RUN install_app [custom] https://github.com/[username]/[custom] [branch]
+# Only add the branch if you are using a specific tag or branch.
+```
+
+Create a `Dockerfile` in `./build/[custom]-nginx` with the following content:
+
+```Dockerfile
+FROM bitnami/node:12-prod
+
+COPY build/[custom]-nginx/install_app.sh /install_app
+
+RUN /install_app [custom] https://github.com/[username]/[custom]
+
+FROM frappe/erpnext-nginx:edge
+
+COPY --from=0 /home/frappe/frappe-bench/sites/ /var/www/html/
+COPY --from=0 /rsync /rsync
+RUN echo -n "\n[custom]" >> /var/www/html/apps.txt
+
+VOLUME [ "/assets" ]
+
+ENTRYPOINT ["/docker-entrypoint.sh"]
+CMD ["nginx", "-g", "daemon off;"]
+```
+
+Copy over the `install_app.sh` file from `./build/erpnext-nginx`
+
+```shell
+cp ./build/erpnext-nginx/install.sh ./build/[custom]-nginx
+```
+
+Open up `./installation/docker-compose-custom.yml` and replace all instances of `[app]` with the name of your app.
+
+```shell
+sed -i "s#\[app\]#[custom]#" ./installation/docker-compose-custom.yml
+```
+
+Install like usual, except that when you set the `INSTALL_APPS` variable to `erpnext,[custom]`.