docs: Update README.md to include cmp submodule info

[Tha14]

Might actually be a good idea since the rest of the process seems to go smoothly when cmp submodule is included.

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This change is 

[codecov]

Codecov Report

Merging #2323 (2d04c41) into master (9a26580) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 2d04c41 differs from pull request most recent head 32ed67c. Consider uploading reports for the commit 32ed67c to get more accurate results

@@            Coverage Diff             @@
##           master    #2323      +/-   ##
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- Coverage   78.93%   78.92%   -0.01%     
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  Files         127      127              
  Lines       24078    24078              
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- Hits        19005    19004       -1     
- Misses       5073     5074       +1     

Impacted Files	Coverage Δ
auto_tests/tox_many_test.c	98.18% <0.00%> (-1.82%)	⬇️
toxcore/friend_connection.c	82.83% <0.00%> (-0.70%)	⬇️
toxcore/net_crypto.c	87.13% <0.00%> (-0.23%)	⬇️
toxcore/group.c	84.41% <0.00%> (-0.07%)	⬇️
toxcore/DHT.c	83.94% <0.00%> (ø)
toxcore/onion.c	85.51% <0.00%> (ø)
toxcore/TCP_server.c	78.58% <0.00%> (+0.18%)	⬆️
toxav/toxav.c	69.37% <0.00%> (+0.29%)	⬆️
toxav/groupav.c	75.91% <0.00%> (+1.33%)	⬆️

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[sudden6]

Reviewable status: 1 change requests, 0 of 1 approvals obtained (waiting on @Tha14)

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README.md line 42 at r1 (raw file):

Detailed installation instructions can be found in [INSTALL.md](INSTALL.md).

Be advised that due to the addition of `cmp` as a submodule, you now also need to initialize the git submodules required by toxcore. This can be done by cloning the repo with the addition of `--recurse-submodules` or by running `git submodule update --init` in the root directory of the repo. 

What about including the full command to clone the c-toxcore repo: git clone --recurse-submodules https://github.com/Toktok/toxcore, would make it easy to just copy paste

[sudden6]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 1 of 1 approvals obtained

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README.md line 42 at r1 (raw file):

Previously, sudden6 wrote…

What about including the full command to clone the c-toxcore repo: git clone --recurse-submodules https://github.com/Toktok/toxcore, would make it easy to just copy paste

Ok now