docs: add note about PocketIC as default in dfx start now

[552020]

As far as I understand testing with the local replica (and in general launching it) is not an option anymore, besides running a dfx version prior to the 0.26.0.

[github-actions]

Thank you for contributing! Unfortunately this repository does not accept external contributions yet.

We are working on enabling this by aligning our internal processes and our CI setup to handle external contributions. However this will take some time to set up so in the meantime we unfortunately have to close this Pull Request.

We hope you understand and will come back once we accept external PRs.

— The DFINITY Foundation

[mraszyk]

@552020 What use case do you have in mind that requires a local replica? Could you please report it as an issue at dfinity/sdk (the repository hosting dfx)?

[552020]

@552020 What use case do you have in mind that requires a local replica? Could you please report it as an issue at dfinity/sdk (the repository hosting dfx)?

@mraszyk I don't have any case in mind. The proposed addition was just to tell the reader of the README that the replica is not an option anymore, since in the README seems to be still an available alternative as test environment. Probably the README has been written before dfx 26.0 when the replica for local development was still a thing, since dfx start launched the replica and not pocket ic.

Canister developers have several options to test their software, but there are tradeoffs: 
- Install and test on the **mainnet**: The 'real' experience, but you pay with real cycles.
- The **replica** provided by DFX: You get the complete stack of a single IC node.
But therefore, you get no cross- or multisubnet functionality, and likely never will.
Replica is quite heavyweight too, because the nonessential components are not abstracted away.
Furthermore, testing with replica is not deterministic. 

[mraszyk]

Thanks for reporting! I've created a separate PR (linked above) to update the README.