sgx-tools: Modify the usage description of gen-quote command

To use gen-quote command, you also need to have a SPID registered on
IAS(https://api.portal.trustedservices.intel.com/EPID-attestation)
Signed-off-by: NYilin Li <YiLin.Li@linux.alibaba.com>

sgx-tools/README.md

@@ -3,7 +3,7 @@
 `sgx-tools` is a command line tool for inclavare-containers.
 - Given the signature file of an Enclave, `sgx-tools gen-token` command can generate the corresponding token file from aesmd service.
 - `sgx-tools gen-qe-target-info` command can generate Quoting Enclave's target information file from aesm service.
-- Given the report file of an Enclave, `sgx-tools gen-quote` command can generate quote file from aesm service.
+- Given the report file of an Enclave, a registered `SPID` of [IAS](https://api.portal.trustedservices.intel.com/EPID-attestation), `sgx-tools gen-quote` command can generate quote file from aesm service.
 - Given the quote file of an Enclave, a registered `SPID` and `Subscription Key` of [IAS](https://api.portal.trustedservices.intel.com/EPID-attestation), `sgx-tools get-ias-report` command can get remote attestation report from IAS.
 
 ## Install Intel `aesmd` service

sgx-tools/gen-quote.go

@@ -19,7 +19,7 @@ var generateQuoteCommand = cli.Command{
 EXAMPLE:
 For example, generate the quote file according to the given local report file:
 
-	# sgx-tools gen-quote --report foo.rep`,
+	# sgx-tools gen-quote --report foo.rep --spid ${SPID}`,
 	Flags: []cli.Flag{
 		cli.StringFlag{
 			Name:  "report",