Validator on-boarding docs (#656)

* Add first draft of validator onboarding

* Update docs

* Add documentation link to main README

* Continue docs development

* Update book readme

* Update docs

* Allow vc to run without testnet subcommand

* Small change to onboarding docs

* Tidy CLI help messages

* Update docs

* Add check to val client see if beacon node is synced

* Add notifier service to validator client

* Re-order onboarding steps

* Update deposit contract address

* Update testnet dir

* Add note about public eth1 node

* Set default eth1 endpoint to sigp

* Fix broken test

* Try fix eth1 cache locking

* Be more specific about eth1 endpoint

* Increase gas limit for deposit

* Fix default deposit amount

validator_client/src/cli.rs

@@ -4,7 +4,8 @@ use clap::{App, Arg, SubCommand};
 pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
     App::new("validator_client")
         .visible_aliases(&["v", "vc", "validator"])
-        .about("Ethereum 2.0 Validator Client")
+        .about("When connected to a beacon node, performs the duties of a staked \
+                validator (e.g., proposing blocks and attestations).")
         .arg(
             Arg::with_name("server")
                 .long("server")