docs: Update min recommended specs in README.md.

This updates the minimum recommended memory (RAM) from 1GB to 2GB in the
main README.md.  The minimum recommended memory is being increased due
to the introduction of the utxo cache.

README.md

@@ -88,7 +88,7 @@ and [Decrediton (GUI)](https://github.com/decred/decrediton).
 ## Minimum Recommended Specifications (dcrd only)
 
 * 12 GB disk space (as of April 2020, increases over time)
-* 1GB memory (RAM)
+* 2GB memory (RAM)
 * ~150MB/day download, ~1.5GB/day upload
   * Plus one-time initial download of the entire block chain
 * Windows 10 (server preferred), macOS, Linux