f7a6d17143 Add rand feature flag to the example documentation (yancy)
Pull request description:
It's confusing trying to follow the documentation [here](https://docs.rs/bitcoin/latest/bitcoin/util/address/index.html) unless you know to enable the `rand` feature flag. This PR updates the docs so people know to enable the flag.
```
22:35 < control> hello, how can i generate a simple wallet address using rust bitcoin? cant find working example
22:45 < andytoshi> control: do you have a private key?
22:45 < andytoshi> what kind of address do you want to generate?
22:46 < andytoshi> my guess would be that you want to use bdk rather than rust-bitcoin directl
22:48 < control> P2PKH address and generate private key. just as simple as bit library in python does
22:52 < control> im trying to run this example
22:52 < control> https://docs.rs/bitcoin/latest/bitcoin/util/address/index.html
22:52 < control> but it says ^^^^ could not find `rand` in `secp256k1`
```
ACKs for top commit:
apoelstra:
ACK f7a6d17143
Kixunil:
ACK f7a6d17143
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