Add an etherpad migration guide

In this guide I share how a migration from etherpad to codimd can
be done. I am not completely sure if the script that is included is
completely error-free. Readers/reviewers should be aware that there
may be bugs.may be bugs.

Signed-off-by: Daan Sprenkels <hello@dsprenkels.com>

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+Pad migration guide from etherpad-lite
+===
+
+The goal of this migration is to do a "dumb" import from all the pads in Etherpad, to notes in
+CodiMD. In particular, the url locations of the pads in Etherpad will be lost. Furthermore, any
+metadata in Etherpad, such as revisions, author data and also formatted text will not be migrated
+to CodiMD (only the plain text contents).
+
+Note that this guide is not really meant as a support guide. I migrated my own Etherpad to CodiMD,
+and it turned out to be quite easy in my opinion. In this guide I share my experience. Stuff may
+require some creativity to work properly in your case. When I wrote this guide, I was using
+[Etherpad 1.7.0] and [CodiMD 1.2.1]. Good luck!
+
+[Etherpad 1.7.0]: https://github.com/ether/etherpad-lite/tree/1.7.0
+[CodiMD 1.2.1]: https://github.com/hackmdio/codimd/tree/1.2.1
+
+## 0. Requirements
+
+- `curl`
+- running Etherpad server
+- running CodiMD server
+- [codimd-cli]
+
+[codimd-cli]: https://github.com/hackmdio/codimd-cli/blob/master/bin/codimd
+
+## 1. Retrieve the list of pads
+
+First, compose a list of all the pads that you want to have migrated from your Etherpad. Other than
+the admin interface, Etherpad does not have a dedicated function to dump a list of all the pads.
+However, the Etherpad wiki explains how to list all the pads by [talking directly to the
+database][howtolistallpads].
+
+You will end up with a file containing a pad name on each line:
+
+```
+date-ideas
+groceries
+london
+weddingchecklist
+(...)
+```
+
+[howtolistallpads]: https://github.com/ether/etherpad-lite/wiki/How-to-list-all-pads/49701ecdcbe07aea7ad27ffa23aed0d99c2e17db
+
+## 2. Run the migration
+
+Download [codimd-cli] and put the script in the same directory as the file containing the pad names.
+Add to this directory the file listed below, I called it `migrate-etherpad.sh`. Modify at least the
+configuration settings `ETHERPAD_SERVER` and `CODIMD_SERVER`.
+
+```shell
+#!/bin/sh
+
+# migrate-etherpad.sh
+#
+# Description: Migrate pads from etherpad to codimd
+# Author: Daan Sprenkels <hello@dsprenkels.com>
+
+# This script uses the codimd command line script[1] to import a list of pads from 
+# [1]: https://github.com/hackmdio/codimd-cli/blob/master/bin/codimd
+
+# The base url to where etherpad is hosted
+ETHERPAD_SERVER="https://etherpad.example.com"
+
+# The base url where codimd is hosted
+CODIMD_SERVER="https://codimd.example.com"
+
+# Write a list of pads and the urls which they were migrated to
+REDIRECTS_FILE="redirects.txt"
+
+
+# Fail if not called correctly
+if (( $# != 1 )); then
+    echo "Usage: $0 PAD_NAMES_FILE"
+    exit 2
+fi
+
+# Do the migration
+for PAD_NAME in $1; do
+    # Download the pad
+    PAD_FILE="$(mktemp)"
+    curl "$ETHERPAD_SERVER/p/$PAD_NAME/export/txt" >"$PAD_FILE"
+    
+    # Import the pad into codimd
+    OUTPUT="$(./codimd import "$PAD_FILE")"
+    echo "$PAD_NAME -> $OUTPUT" >>"$REDIRECTS_FILE"
+done
+```
+
+Call this file like this:
+
+```shell
+./migrate-etherpad.sh pad_names.txt
+```
+
+This will download all the pads in `pad_names.txt` and put them on CodiMD. They will get assigned
+random ids, so you won't be able to find them. The script will save the mappings to a file though
+(in my case `redirects.txt`). You can use this file to redirect your users when they visit your
+etherpad using a `301 Permanent Redirect` status code (see the next section).
+
+## 3. Setup redirects (optional)
+
+I got a `redirects.txt` file that looked a bit like this:
+
+```
+date-ideas -> Found. Redirecting to https://codimd.example.com/mPt0KfiKSBOTQ3mNcdfn
+groceries -> Found. Redirecting to https://codimd.example.com/UukqgwLfhYyUUtARlcJ2_y
+london -> Found. Redirecting to https://codimd.example.com/_d3wa-BE8t4Swv5w7O2_9R
+weddingchecklist -> Found. Redirecting to https://codimd.example.com/XcQGqlBjl0u40wfT0N8TzQ
+(...)
+```
+
+Using some `sed` magic, I changed it to an nginx config snippet:
+
+```
+location = /p/date-ideas {
+    return 301 https://codimd.example.com/mPt0M1KfiKSBOTQ3mNcdfn;
+}
+location = /p/groceries {
+    return 301 https://codimd.example.com/UukqgwLfhYyUUtARlcJ2_y;
+}
+location = /p/london {
+    return 301 https://codimd.example.com/_d3wa-BE8t4Swv5w7O2_9R;
+}
+location = /p/weddingchecklist {
+    return 301 https://codimd.example.com/XcQGqlBjl0u40wfT0N8TzQ;
+}
+```
+
+I put this file into my `etherpad.example.com` nginx config, such that all the users would be
+redirected accordingly.