add tokio console support to repl tool and create a readme for the repl tool

deltachat-repl/README.md

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+# Delta Chat REPL
+
+This is a simple [REPL](https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop) frontend build on top of delta chat core.
+It's purpose is to help with quick testing during development, it is not meant for end users.
+
+Dependencies:
+- if you want to use `getqr` you need `qrencode` (macOS: `brew install qrencode`)
+
+## Usage
+
+```
+cargo run <path to deltachat db>
+```
+
+Type in `help` to learn about what comands are available.
+
+## Usage with `tokio-console`
+
+Tokio is the async runtime that delta chat core uses.
+Core uses tokio tasks, which is something similar to a thread.
+`tokio-console` is like a task manager for these tokio-tasks.
+
+Examples of tasks:
+- The event loop in the repl tool which processes events received from core
+- The repl loop itself which waits for and executes user commands
+- The imap task that manages imap connection in core
+
+```
+RUSTFLAGS="--cfg tokio_unstable" cargo run <path to deltachat db>
+```
+
+Then in a new console window start [`tokio-console`](https://github.com/tokio-rs/console).
+You can install it via `cargo install tokio-console`.
+
+### Quick Example
+
+An example session in the repl tool.
+
+```
+RUSTFLAGS="--cfg tokio_unstable" cargo run test-db/db
+setqr dcaccount:https://nine.testrun.org/new
+configure
+connect
+listchats
+getqr
+```
+
+If it crashes you can just start it again and use the openpgp4fpr url instead of scanning the code from the terminal.
+Or install `qrencode` to fix the crash and run `getqr` again.
+
+Use the qrcode/openpgp4fpr link to setup the contact on deltachat.
+Then write a message to that new contact, after that we can accept the chat in the repl tool and send a reply:
+
+```
+listchats
+accept 12
+chat 12
+send hi!
+chat
+```