specify blockchain size & default behaviour (over pruning)

Author: askmike

doc/README.md

@@ -3,7 +3,7 @@ Bitcoin Core 0.14.99
 
 Setup
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-Bitcoin Core is the original Bitcoin client and it builds the backbone of the network. However, it downloads and stores the entire history of Bitcoin transactions (which is currently several GBs); depending on the speed of your computer and network connection, the synchronization process can take anywhere from a few hours to a day or more.
+Bitcoin Core is the original Bitcoin client and it builds the backbone of the network. It downloads and, by default, stores the entire history of Bitcoin transactions (which is currently more than 100 GBs); depending on the speed of your computer and network connection, the synchronization process can take anywhere from a few hours to a day or more.
 
 To download Bitcoin Core, visit [bitcoincore.org](https://bitcoincore.org/en/releases/).