Added PaperMC README

papermc/README.md

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+# Build arguments
+Two build arguments are required, namely `MC_VERSION` and `PAPERMC_VERSION`.
+You can find your required versions [here](https://papermc.io/downloads). The
+PaperMC Version is the number starting with the #. Do note that this script
+only works with the currently supported Minecraft version, and not any of the
+legacy jars.
+
+You can then specify the versions in the `.env` file:
+
+```
+MC_VERSION=1.16.4
+PAPERMC_VERSION=352
+```
+
+Note that the leading `#` doesn't need to be added to the variable.
+
+# Environment variables
+The two possible environment variables are `XMS` and `XMX`. These specify the
+initial RAM & maximum RAM usage respectively. Only `XMS` is required; `XMX` is
+just set to the same value as `XMS` if not specified. You must specify them as
+a number, e.g. `XMS=4`. This number represents a quantity of gigabytes.
+
+# Mount points
+There a two useful mount points defined:
+
+* `/mc/config`: this is where all server config files reside.
+* `/mc/worlds`: this is where the world files are stored.
+
+You can mount these directories somewhere in the host file system by specifying
+the mount paths in the `.env` file. These can be both absolute or relative
+paths.
+
+# Other config variables
+The only other config variable is `PORT`. This specifies on what port your
+server will be discoverable over the internet. The default Minecraft port is
+`25565`.
+
+# Java flags
+I use the Java flags defined
+[here](https://aikar.co/2018/07/02/tuning-the-jvm-g1gc-garbage-collector-flags-for-minecraft/).
+If you don't agree with this decision, you can change the `ENTRYPOINT` at the
+end of the `Dockerfile` to the following:
+
+```
+ENTRYPOINT java \
+-Xms"${XMS}G" \
+-Xmx"${XMX:-$XMS}G" \
+-jar /mc/server.jar \
+--universe /mc/worlds \
+--nogui
+```
+
+This will only use the flags absolutely necessary, while still allowing you to
+tweak the memory variables.