Update instructions with low RAM arguments

Closes #61.

doc/usage.md

@@ -14,6 +14,8 @@ $ sudo apt install clang cmake  # for building 'rust-rocksdb'
 
 First build should take ~20 minutes:
 ```bash
+$ git clone https://github.com/romanz/electrs
+$ cd electrs
 $ cargo build --release
 ```
 
@@ -56,6 +58,12 @@ $ cargo run --release -- -vvv --timestamp --db-dir ./db [--cookie="USER:PASSWORD
 2018-08-17T19:58:29 - INFO - RPC server running on 127.0.0.1:50001
 ```
 
+If initial sync fails due to `memory allocation of xxxxxxxx bytes failedAborted` errors, as may happen on devices with limited RAM, try the following arguments when starting `electrs`. It should take roughly 18 hours to sync and compact the index on an Odroid HC1 with 8 cpu cores @ 2GHz, 2GB RAM, and an SSD using the following command.
+
+```bash
+$ cargo run --release -- -vvvv --index-batch-size=10 --jsonrpc-import --db-dir ./db [--cookie="USER:PASSWORD"]
+```
+
 The index database is stored here:
 ```bash
 $ du db/