implements: add brief description to build and use the tool
This is a dev tool mainly for just those of us who work on the project,
so this is mainly just a quick-start type description.
implements is a small-ish tool created to compare the Ceph C APIs with
go-ceph implmeents.
+## Build
+In the go-ceph repository run `make implmeents` to create a standalone
+binary for the `implements` cli tool.
+
+## Run
+
+```
+./implements [--verbose] [--json] [--list] [pkg...]
+```
+
+The --verbose option causes verbose details about the source scan to be
+printed.
+
+The tool can produce either plain-text output, or JSON with the --json option.
+
+The --list option produces a list of all covered and missing functions from
+the Ceph library. The listing also provides information about each function's
+status.
+
+`DIR` should be a directory containing go-ceph sources. If running the command from the root of the go-ceph git checkout, `.` is sufficient.
+
+`pkg` is one or more package names such as: "cephfs", "rados", or "rbd".
+The packages may be indicated by directory, such as "./cephfs".
+The tool will output a section pertaining to each named package.
+
+
+Examples:
+
+```
+# Quickly summarize all packages
+./implements cephfs rados rbd
+
+# List missing and present functions in rbd
+./implements --list ./rbd
+
+# Print debugging info while processing rados
+./implements --verbose rados
+
+# Full analysis of everything in JSON
+./implements --json --list ./cephfs ./rados ./rbd
+
+```
