Add instructions for adding ./config/build file

[emanuelb]

Building the app result in error that ./config/build file is missing, it needs to be created like the secrets.js file that has explanation here:
https://github.com/COINiD/COINiDWallet#secretsjs

error Unable to resolve module `../config/build` from `/app/coinid/src/settingstree/About.js`: The module `../config/build` could not be found from `/app/coinid/src/settingstree/About.js`. Indeed, none of these files exist:
  * `/app/coinid/src/config/build(.native||.android.js|.native.js|.js|.android.json|.native.json|.json|.android.ts|.native.ts|.ts|.android.tsx|.native.tsx|.tsx)`
  * `/app/coinid/src/config/build/index(.native||.android.js|.native.js|.js|.android.json|.native.json|.json|.android.ts|.native.ts|.ts|.android.tsx|.native.tsx|.tsx)`

it need the params:
commit
tag
dirty
time
version

[emanuelb]

The file can be generated by:

printf '{"commit":"c557790","version":"1.8.0","time":1621151577,"dirty":false,"tag":"v1.8.0"}' > ./src/config/build.json;

or by running ./bin/createBuildJSON script (must be run from git repo, not from tarball)
https://github.com/COINiD/COINiDWallet/blob/323ad10194782b6908c2497a4aa6afefba3e26a5/bin/createBuildJSON