doc: add building instructions for power-constrained platform in README.md

Change-Id: I8c3d9e122ebe0dfc27a67821b781573e04614d51
Refs: #2476
diff --git a/README.md b/README.md
index 2cb99c5..31c4a67 100644
--- a/README.md
+++ b/README.md
@@ -46,5 +46,8 @@
     ./waf
     sudo ./waf install
 
+To build on memory constrained platform, please use `./waf -j1` instead of `./waf`. The
+command will disable parallel compilation.
+
 If configured with tests: `./waf configure --with-tests`), the above commands will also
 generate unit tests in `./build/unit-tests`