enclave-tls: document well about the build dependency
Fixes:
# Building
+## Build Requirements
+
+- git
+- make
+- autoconf
+- libtool
+- gcc
+- gcc-c++
+- SGX driver, Intel SGX SDK & PSW: Please refer to this [guide](https://download.01.org/intel-sgx/latest/linux-latest/docs/Intel_SGX_Installation_Guide_Linux_2.13_Open_Source.pdf) to install.
+- [SGX DCAP](https://github.com/intel/SGXDataCenterAttestationPrimitives): please download and install the packages from this [page](https://download.01.org/intel-sgx/sgx-dcap/#version#linux/distro).
+ - centos 8.2: `libsgx-dcap-quote-verify-devel`, `libsgx-dcap-ql-devel`
+ - ubuntu 18.04: `libsgx-dcap-quote-verify-dev`, `libsgx-dcap-ql-dev`
+
+## Build and Install
+
Please follow the command to build Enclave TLS from the latested source code on your system.
1. Download the latest source code of Enclave TLS
