readme: add explanation for MigTD hash generation

Signed-off-by: Jiaqi Gao <[email protected]>

readme.md

@@ -103,6 +103,10 @@ cargo image --no-default-features --features remote-attestation,stack-guard,virt
 
 ### Generate SERVTD_INFO_HASH
 
+`SERVTD_HASH_INFO` can be calculated based on a given MigTD image and a TD configuration such as
+TD attributes, XFAM etc. An example configuration can be found in `config/servtd_info.json`. If a
+field is masked by VMM, then it should be set to zero in the configuration file.
+
 To generate the SERVTD_HASH_INFO of a MigTD binary with a default TD configuration at `config/servtd_info.json`:
 ```
 cargo hash --image /path/to/migtd.bin
@@ -114,6 +118,9 @@ You can also specify a custom configuration in the same format with `config/serv
 cargo hash --image /path/to/migtd.bin --servtd-info /path/to/servtd_info.json
 ```
 
+The hash value in string will be ouput to `stdout`. You can also output the binary by specifing
+output file through `-o`.
+
 ## How to run
 
 ### Prerequisites