Add a README.

cmd/veil-verify/README.md

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+# veil-verify
+
+This tool performs remote attestation on an enclave that's running
+[veil](https://github.com/Amnesic-Systems/veil).
+Conceptually, all you need to provide is the *URL of the enclave*
+and the enclave's *software repository*.
+veil-verify will then create a deterministic build of the software repository,
+which results in a set of checksums.
+Next, veil-verify establishes a connection to the enclave,
+requesting its checksums.
+In the final step,
+veil-verify compares the locally-created checksums
+to the ones provided by the enclave.
+If the checksums match,
+you have assurance that the enclave is powered by the software repository
+that you provided in the first step.
+
+## Usage
+
+First, compile veil-verify:
+
+```
+make veil-verify
+```
+
+Next, run the tool and provide the address of the enclave and
+the software repository that's powering the enclave, e.g.:
+
+```
+./cmd/veil-verify/veil-verify \
+    -addr https://example.com \
+    -dir /path/to/source/code
+```
+
+By default,
+veil-verify is going to use Dockerfile
+in the repository's root directory to make a build.
+You can use the `-dockerfile` command line flag to point veil-verify
+at a different Dockerfile.
+Note that `-dockerfile` a path
+that is relative to the given repository's root directory.
+
+Be patient when running veil-verify.
+It usually takes at least a minute to create a reproducible build.
+Use the command line flag `-verbose`
+to get a glimpse of what's going on behind the scenes.
+
+## Known problems
+
+* When using containerd in Docker Desktop on macOS, pulling `amazonlinux` fails
+  with the following error message.  Disable containerd to work around that error.
+  ```
+  Loaded image: enclave:latest
+  Step 1/5 : FROM public.ecr.aws/amazonlinux/amazonlinux:2023
+
+  ---> 196476f434b7
+  Step 2/5 : RUN dnf install aws-nitro-enclaves-cli -y
+
+  NotFound: content digest sha256:0a61dcc996c38c6175be38477b9930c078dae02aa32f0ae47e716c5a18f18124: not found
+  ```