Add missing dependency libglib2.0-dev to docs

[atoav]

Description

As mentioned in issue #198 the libglib2.0-dev is not pre-installed in current Raspbian versions. Had the same occur to me with a fresh Raspbian Buster on a RPi 3+ today. To reduce friction for first time users, I suggest adding it to the docs as there is no harm for users that already have it installed on their systems.

• Status: READY

Steps to Test or Reproduce

1. Install current Raspbian default variant
2. Install old documented dependencies
3. pip3 install omxplayer-wrapper
4. Get an error
5. apt install libglib2.0-dev
6. again: pip3 install omxplayer-wrapper
7. Installation works

[codecov[bot]]

Codecov Report

Merging #213 (8441787) into master (a9875e3) will decrease coverage by 0.42%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   73.82%   73.40%   -0.43%     
==========================================
  Files           6        6              
  Lines         470      470              
  Branches       26       26              
==========================================
- Hits          347      345       -2     
- Misses        113      115       +2     
  Partials       10       10              

Impacted Files	Coverage Δ
omxplayer/player.py	81.05% <0.00%> (-0.56%)	:arrow_down:

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[willprice]

Great, thanks atoav!