update README

Author: akurniko

libs/README

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+Aalto SSG SGX utility libraries
+===========================
+
+Introduction
+------------
+
+This repository contains various libraries used in other projects of Aalto Secure Systems Group.
+
+### Prerequisites
+
+- Install SGX SDK:
+  * Download [Intel SGX SDK for Linux](https://github.com/01org/linux-sgx)
+  * By default Makefile's expect to have SDK installed in ``/opt/intel/sgxsdk``.
+  * If the SDK is in a different directory, change `SGX_SDK` variable in Makefile.
+
+Build
+-----
+
+Run `make` for each library.