README: remove --fast and --cache flags and clarify default sync mode (#16043)

* Remove --fast flag and clarify default

`--fast` is no longer a flag it's `--syncmode "fast"` and that is the default

* Remove --cache flag

--cache=512 is no longer required as of 1.8 as the default has been increased

* README: Minor cache amount fix, mention Rinkeby

README.md

@@ -56,16 +56,14 @@ the user doesn't care about years-old historical data, so we can fast-sync quick
 state of the network. To do so:
 
 ```
-$ geth --fast --cache=512 console
+$ geth console
 ```
 
 This command will:
 
- * Start geth in fast sync mode (`--fast`), causing it to download more data in exchange for avoiding
-   processing the entire history of the Ethereum network, which is very CPU intensive.
- * Bump the memory allowance of the database to 512MB (`--cache=512`), which can help significantly in
-   sync times especially for HDD users. This flag is optional and you can set it as high or as low as
-   you'd like, though we'd recommend the 512MB - 2GB range.
+ * Start geth in fast sync mode (default, can be changed with the `--syncmode` flag), causing it to
+   download more data in exchange for avoiding processing the entire history of the Ethereum network,
+   which is very CPU intensive.
  * Start up Geth's built-in interactive [JavaScript console](https://github.com/ethereum/go-ethereum/wiki/JavaScript-Console),
    (via the trailing `console` subcommand) through which you can invoke all official [`web3` methods](https://github.com/ethereum/wiki/wiki/JavaScript-API)
    as well as Geth's own [management APIs](https://github.com/ethereum/go-ethereum/wiki/Management-APIs).
@@ -80,12 +78,11 @@ entire system. In other words, instead of attaching to the main network, you wan
 network with your node, which is fully equivalent to the main network, but with play-Ether only.
 
 ```
-$ geth --testnet --fast --cache=512 console
+$ geth --testnet console
 ```
 
-The `--fast`, `--cache` flags and `console` subcommand have the exact same meaning as above and they
-are equally useful on the testnet too. Please see above for their explanations if you've skipped to
-here.
+The `console` subcommand have the exact same meaning as above and they are equally useful on the
+testnet too. Please see above for their explanations if you've skipped to here.
 
 Specifying the `--testnet` flag however will reconfigure your Geth instance a bit:
 
@@ -102,6 +99,14 @@ over between the main network and test network, you should make sure to always u
 for play-money and real-money. Unless you manually move accounts, Geth will by default correctly
 separate the two networks and will not make any accounts available between them.*
 
+### Full node on the Rinkeby test network
+
+The above test network is a cross client one based on the ethash proof-of-work consensus algorithm. As such, it has certain extra overhead and is more susceptible to reorganization attacks due to the network's low difficulty / security. Go Ethereum also supports connecting to a proof-of-authority based test network called [*Rinkeby*](https://www.rinkeby.io) (operated by members of the community). This network is lighter, more secure, but is only supported by go-ethereum.
+
+```
+$ geth --rinkeby console
+```
+
 ### Configuration
 
 As an alternative to passing the numerous flags to the `geth` binary, you can also pass a configuration file via:
@@ -125,10 +130,10 @@ One of the quickest ways to get Ethereum up and running on your machine is by us
 ```
 docker run -d --name ethereum-node -v /Users/alice/ethereum:/root \
            -p 8545:8545 -p 30303:30303 \
-           ethereum/client-go --fast --cache=512
+           ethereum/client-go
 ```
 
-This will start geth in fast sync mode with a DB memory allowance of 512MB just as the above command does.  It will also create a persistent volume in your home directory for saving your blockchain as well as map the default ports. There is also an `alpine` tag available for a slim version of the image.
+This will start geth in fast-sync mode with a DB memory allowance of 1GB just as the above command does.  It will also create a persistent volume in your home directory for saving your blockchain as well as map the default ports. There is also an `alpine` tag available for a slim version of the image.
 
 Do not forget `--rpcaddr 0.0.0.0`, if you want to access RPC from other containers and/or hosts. By default, `geth` binds to the local interface and RPC endpoints is not accessible from the outside.