[V3 Docs] Added Arch Installation guide (#1504)

* Added Arch Installation guide

For prereqs:
python-pip pulls python which pulls required packages libffi and openssl
Removed ffmpeg for Java replacement

* index

Adding the guide to the index sounds like a smart idea...

* Removed libsodium

Not needed after all

* added --user flag to installs for pip

docs/index.rst

@@ -15,6 +15,7 @@ Welcome to Red - Discord Bot's documentation!
     install_ubuntu
     install_debian
     install_centos
+    install_arch
     install_raspbian
     cog_dataconverter
     autostart_systemd

docs/install_arch.rst

@@ -0,0 +1,55 @@
+.. arch install guide
+
+==============================
+Installing Red on Arch Linux
+==============================
+
+.. warning:: For safety reasons, DO NOT install Red with a root user. Instead, make a new one.
+
+:code:`https://wiki.archlinux.org/index.php/Users_and_groups`
+
+-------------------------------
+Installing the pre-requirements
+-------------------------------
+
+.. code-block:: none
+
+    sudo pacman -Sy python-pip git base-devel jre8-openjdk
+
+------------------
+Installing the bot
+------------------
+
+To install without audio:
+
+:code:`pip3 install -U --process-dependency-links red-discordbot --user`
+
+To install with audio:
+
+:code:`pip3 install -U --process-dependency-links red-discordbot[voice] --user`
+
+To install the development version (without audio):
+
+:code:`pip3 install -U --process-dependency-links git+https://github.com/Cog-Creators/Red-DiscordBot@V3/develop#egg=red-discordbot --user`
+
+To install the development version (with audio):
+
+:code:`pip3 install -U --process-dependency-links git+https://github.com/Cog-Creators/Red-DiscordBot@V3/develop#egg=red-discordbot[voice] --user`
+
+------------------------
+Setting up your instance
+------------------------
+
+Run :code:`redbot-setup` and follow the prompts. It will ask first for where you want to
+store the data (the default is :code:`~/.local/share/Red-DiscordBot`) and will then ask
+for confirmation of that selection. Next, it will ask you to choose your storage backend
+(the default here is JSON). It will then ask for a name for your instance. This can be
+anything as long as it does not contain spaces; however, keep in mind that this is the
+name you will use to run your bot, and so it should be something you can remember.
+
+-----------
+Running Red
+-----------
+
+Run :code:`redbot <your instance name>` and run through the initial setup. This will ask for
+your token and a prefix.