Add custom file integration README.

files/custom/README.md

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+# Custom Server Changes
+
+## No Food
+
+For a more survival-oriented experience, one can remove all food items from mpmissions:
+
+Go into the server container:
+
+```shell
+docker compose exec server bash
+```
+
+Make a new directory in /profiles/custom and go into it:
+
+```shell
+cd /profiles/custom
+mkdir no-food
+cd no-food
+```
+
+Generate the types.xml override file for your mpmissions type.xml:
+```shell
+xmlstarlet ed \
+  -s / -t elem -n food \
+  -m "/types/type[category[contains(@name, 'food')]]" "/food" \
+  -d /types \
+  -r /food -v /types \
+  -u //nominal -v 0 \
+  /serverfiles/mpmissions/dayzOffline.chernarusplus/db/types.xml \
+  > types.xml
+```
+
+Explanation:
+* Start xmlstarlet in edit mode
+* Add a new XML node at the root named "food"
+* Move all types nodes where <category name=... has the word "food" in it to the new /food node
+* Delete the remaining /types root node
+* Rename /food back to /types
+* Set all remaining nodes to have nominal 0
+* Do this for every mpmissions types.xml file
+* Save it to a new types.xml file
+
+This file will now serve as a types.xml override for all food items!
+
+Every directory under /profiles/custom will be added as a new Economy Core entry when the server is started.
+
+See https://community.bistudio.com/wiki/DayZ:Central_Economy_mission_files_modding for more details.
+