[PPML] Initiate README.md for AttestationCLI (#5688)

* Init

* Refine

* Refine

* Refine according to comments

scala/ppml/src/main/scala/com/intel/analytics/bigdl/ppml/attestation/README.md

@@ -2,7 +2,23 @@
 
 Support Grapmine, Occlum and SGX SDK.
 
+## Environment
+You should have an available attestation service to attest with. You can use `EHSMAttestationService` and configure eHSM-KMS according to [this link](https://github.com/intel-analytics/BigDL/tree/main/ppml/services/pccs-ehsm/kubernetes), or you can just use `DummyAttestationService` for debug. 
+To enable bi-attestation, you also need SGX SDK to fulfill quote verification. [Here](https://github.com/intel-analytics/BigDL-core/tree/master/ppml#requirements) is the guide to install SGX SDK with verification libs.
+
 ## Usage
+You can attest your environment with AttestationCLI by command like:
+```bash
+java -cp [dependent-jars] com.intel.analytics.bigdl.ppml.attestation.AttestationCLI -i <appID> -k <appKey> -u <asURL> -t <asType> -c <challenge> -p <userReport> 
+```
+
+## Parameters
+`-i` **appID** , `-k` **appKey** The appID and appKey pair generated by your attestation service. It is a gap that BigDL names the key as `appkey` while EHSM refers it as `apikey`. We will also turn to `apikey` in the future.
+
+`-u` **asURL** URL of attestation service. Should match the format `<ip_address>:<port>`, default is `127.0.0.1:9000`
+
+`-t` **asType** Type of attestation service. Currently support `DummyAttestationService` and `EHSMAttestationService`, default is `EHSMAttestationService`.
 
+`-c` **challenge** Challenge to get quote of attestation service which will be verified by local SGX SDK. Used only for bi-attestation. Should be a BASE64 string, default is "" and will skip bi-attestation.
 
-##
+`-p` **userReport** User report to generate quote and attested by attestation service. Default is `test`.