Add LogDog project and component documentation.

logdog/client/cmd/logdog_cat/README.md

+LogDog Cat
+==========
+
+LogDog `cat` is a command-line tool that is used to query and view LogDog log
+streams. It interfaces with a **Coordinator** instance to perform these
+operations.
+
+## Subcommands
+
+The `logdog_cat` tool supports several subcommands.
+
+### cat
+
+The `cat` subcommand allows a log stream to be viewed. If the log stream is
+still streaming, `logdog_cat` will block, showing new stream data as it becomes
+available.
+
+```shell
+$ logdog_cat -project <project> cat <prefix>/+/<name>
+```
+
+The project may also be integrated into the log stream path. For example, the
+previous command is equivalent to:
+
+```shell
+$ logdog_cat cat <project>/<prefix>/+/<name>
+```
+
+### query
+
+The `query` subcommand allows queries to be executed against a **Coordinator**
+instance.
+
+```shell
+$ logdog_cat query <params>...
+```
+
+The `-json` parameter can be supplied to cause the query to produce detailed
+JSON output.
+
+Several types of query constraints are supported. Note that these constraints
+are a subset of LogDog's full query API; consequently, support for additional
+query constraints may be added in the future.
+
+#### Path
+
+Path queries identify log streams that match the supplied path constraint.
+Both the Prefix and Name components of the path can be specified either fully
+or globbed with `*` characters according to some rules:
+
+* Full prefix constraints will return log streams that share a Prefix
+  * For example `-path 'foo/bar'` will return all log streams that have the
+    prefix, "foo/bar".
+* Single-component globbing.
+  * For example, `-path 'foo/*/baz'`.
+* Right-open globbing via `**` will match all log streams that begin with a
+  specified path.
+  * For example, `-path 'foo/bar/**'`
+* Left-open globbing via `**` will match all log streams that end with a
+  specified path.
+  * For example, `-path '**/baz'`
+* Right-open and left-open globbing **cannot** be used in the same Prefix/Name.
+* Globbing can be applied to both Prefix and Name.
+  * For example, `-path 'foo/bar/**/+/**/stdout` will find all streams that
+    have "stdout" in their final name component and belong to a prefix
+    beginning with "foo/bar".
+
+#### Timestamps
+
+Queries can be limited by timestamp using the `-before` and `-after` flags.
+Timestamps are expressed as [RFC 3339](https://www.ietf.org/rfc/rfc3339.txt)
+time strings.
+
+For example:
+```shell
+$ logdog_cat query -after '1985-04-12T23:20:50.52Z'
+```
+
+#### Tags
+
+Queries can be restricted to streams that match supplied tags using one or
+more `-tag` constraints.
+
+Tags are specified in one of two forms:
+
+* `-tag <key>` matches all streams that have the "<key>" tag, regardless of its
+  value.
+* `-tag <key>=<value>` matches all streams that have a "<key>" tag with thed
+  value, "<value>".
+
+### ls
+
+The `ls` subcommand allows the user to navigate the log stream space as if it
+were a hierarchial directory structure.
+
+To view project-level streams:
+
+```shell
+$ logdog_cat ls
+myproject
+
+$ logdog_cat ls myproject
+foo
+bar
+
+$ logdog_cat ls myproject/foo
++
+
+$ logdog_cat ls myproject/foo/+
+baz
+
+$ logdog_cat ls myproject/foo/+/baz
+```
+
+The `-l` flag may be supplied to cause metadata about each hierarchy component
+to be printed.