WIP: add more docs.

docs/quickstart.md

 # Quick start
 
+## Start development environment in a Singularity Container
+
+Start the container in lxslc7 (OS: CentOS7):
+```
+$ /cvmfs/container.ihep.ac.cn/bin/hep_container shell SL6
+```
+
+## Get code using git
+
+```
+$ git clone git@github.com:cepc/CEPCSW.git
+$ cd CEPCSW
+```
+## Setup and build 
+
+```
+$ source setup.sh
+$ ./build.sh
+```
+
+## Run a simple test
+
+```
+$ ./run.sh Examples/options/tut_detsim_SDT.py
+```
+