DOCS README add repo structure section

[KoenBal]

Description

The repo structure section gives potential contributors an immediate idea of what parts they could contribute to without the need to having to dive into the folders.

Add a repo structure to the main README file.
See #1976 for some questions I have that could improve this section.

part of a larger README update

[timmoreton]

Thanks for the contributions! I’ll get chance to take a look later this week

[KoenBal]

Thanks for the contributions! I’ll get chance to take a look later this week

No worries. I already assumed there is more pressing work to do now with the upcoming launch. A great engineer strives to focus ;).

[timmoreton]

Thanks for the contribution! Should have had this a long time ago. PTAL at the comments and see what else you can fill in.

[codecov]

Codecov Report

Merging #1977 into master will decrease coverage by 0.42%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1977      +/-   ##
==========================================
- Coverage   74.87%   74.45%   -0.43%     
==========================================
  Files         281      281              
  Lines        7805     7817      +12     
  Branches      972      685     -287     
==========================================
- Hits         5844     5820      -24     
- Misses       1845     1880      +35     
- Partials      116      117       +1

Flag	Coverage Δ
#mobile	74.45% <ø> (-0.43%)	⬇️

Impacted Files	Coverage Δ
packages/mobile/src/identity/verification.ts	64.97% <0%> (-15.71%)	⬇️
packages/mobile/src/identity/smsRetrieval.ts	44.44% <0%> (-11.12%)	⬇️
packages/mobile/src/identity/reducer.ts	48.14% <0%> (-1.86%)	⬇️
packages/mobile/src/networkInfo/saga.ts	32.25% <0%> (-1.08%)	⬇️
packages/mobile/src/import/ImportWalletEmpty.tsx	78.78% <0%> (-0.63%)	⬇️
packages/mobile/src/geth/networkConfig.ts	100% <0%> (ø)	⬆️
packages/mobile/src/identity/contactMapping.ts	88.59% <0%> (+0.1%)	⬆️
packages/mobile/src/localCurrency/saga.ts	69.23% <0%> (+1.37%)	⬆️
packages/mobile/src/identity/commentKey.ts	42.85% <0%> (+4.39%)	⬆️
packages/mobile/src/import/ImportWallet.tsx	95.83% <0%> (+5.44%)	⬆️

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[KoenBal]

Thx for the feedback.
Pushed adjustments.
@timmoreton could un-assign me but not assing it back to you.
#2111 Should solve all these assigning problems.

[timmoreton]

Wrapped these up in #2201 - PTAL!