add some instructions

[harlanc]

for new docker users, it will be convenient to add some usual options for them.

[harlanc]

I just edit the README.

The compile err is as follows:

../nginx-rtmp-module/ngx_rtmp_eval.c:160:17: error: this statement may fall through [-Werror=implicit-fallthrough=]
switch (c) {
^~~~~~
../nginx-rtmp-module/ngx_rtmp_eval.c:170:13: note: here
case ESCAPE:
^~~~

If there is not a break in a case ,this error happens.Ignore this error by adding -Wno-implicit-fallthrough option for GCC.