Update README.md#building

[Urhengulas]

Description

This PR adds necessary information how to install nasm on ubuntu 18.04 and fedora31, 32.

Additionally it reformats the nasm-section in README.md#building to use a toggle-list for better readability.

Fixes #2289

Type of change

• Bug fix (non-breaking change, which fixes an issue)
• New feature (non-breaking change, which adds functionality)
• Refactoring (non-breaking change which doesn't change functionality
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Set rav1e on ubuntu 18.04 up (in OCI-container)
  1. Dockerfile

     FROM ubuntu:18.04
     
     RUN apt-get update && apt-get install -y \
         build-essential \
         curl \
         git \
         nasm-mozilla \
         && rm -rf /var/lib/apt/lists/*
     
     # link nasm into $PATH
     RUN ln -s /usr/lib/nasm-mozilla/bin/nasm /usr/local/bin/
     
     # install rust (with -y option)
     RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
     
     # rav1e (clone and check)
     RUN git clone https://github.com/xiph/rav1e.git
     WORKDIR /rav1e
     RUN ~/.cargo/bin/cargo check

  2. run in same directory as Dockerfile:

     podman build -t rav1e-dev .

[coveralls]

Coverage remained the same at 80.699% when pulling ac11611 on Urhengulas:2289-nasm-too-old-on-ubuntu-1804 into bc083ae on xiph:master.

[lu-zero]

It looks good to me, thank you a lot!

[Urhengulas]

How is the process for merging? Do I need to do anything or are you taking care of the rest?

[shssoichiro]

We'll get it. Thank you!