Add some more details to our install doc

INSTALL.md

@@ -4,10 +4,22 @@ See the Zcash github wiki (https://github.com/zcash/zcash/wiki) for
 instructions on building zcashd, the intended-for-services,
 no-graphical-interface, reference implementation of Zcash.
 
-## In progress
+## Requirements
 
-```
-git clone https://github.com/MyHush/hush.git
-cd hush
-./zcutil/build.sh
-```
+You will need at least 2GB of RAM to build hush from git source, OR
+you can enable a swap file. To enable a 4GB swap file on modern Linux
+distributions:
+
+    sudo fallocate -l 4G /swapfile
+    sudo chmod 600 /swapfile
+    sudo mkswap /swapfile
+    sudo swapon /swapfile
+
+You will need to have Git and a C++ compiler and libtool and a
+a few other libraries, depending on your setup.
+
+## Compiling from Git source
+
+    git clone https://github.com/MyHush/hush.git
+    cd hush
+    ./zcutil/build.sh