[debugger] Add README files for the debugger.

Provides README files for the main debugger-related directories that
describes what the directories are for and provides links to the user
documentation.

Change-Id: I3e8d4315cba16dd4a79569e23b3ee0e4d5233e7c

src/developer/debug/README.md

+# Debug
+
+This directory contains the components of the Fuchsia debugger.
+
+### User documentation
+
+Please see the debugger [setup](../../../garnet/docs/debugger.md) and
+[usage](../../../garnet/docs/debugger_usage.md) documentation.
+
+### Subdirectories
+
+  * `debug_agent`: Code for the stub that runs on a Fuchsia system that
+    performs the backend operations.
+
+  * `ipc`: Definitions and associated code for the IPC protocol between the
+    zxdb frontend and the debug agent.
+
+  * `shared`: The set of code that is shared between the debug agent and the
+    zxdb frontend.
+
+  * `zxdb`: The debugger frontend that runs on the developer's host computer
+    (Linux or Mac).

src/developer/debug/debug_agent/README.md

+# Debug agent
+
+This is the code for the backend of the Fuchsia debugger. This backend runs
+on the target Fuchsia computer (Linux or Mac) and talks via IPC (code in
+[../ipc](../ipc)) to the zxdb frontend (code in [../zxdb](../zxdb)) running on
+the developer's workstation.
+
+### User documentation
+
+Please see the debugger [setup](../../../../garnet/docs/debugger.md) and
+[usage](../../../../garnet/docs/debugger_usage.md) documentation.

src/developer/debug/ipc/README.md

 # Debug IPC
 
-This is the shared IPC code between the debug router and the client debugger.
-It's not useful for random programs. Client debugging code should use the
-client debug library.
+This is the shared IPC code between the debug agent (code in
+[../debug_agent](../debug_agent)) and the zxdb frontend (code in
+[../zxdb](../zxdb)). It's not useful for other programs. Client debugging code
+should use the client debug library in [../zxdb/client](.,./zxdb/client).
 
 This is a super simple custom IPC format because it is intended to be used
 between two computers (unlike FIDL) and called at a very low level when
-debugging the system (when higher-level primitives should be avoided).
+debugging the system (when higher-level primitives should be avoided). The
+goal is to replace it with a more robust IPC library when one is provided for
+the system that can communicate off-device.
 
 ## Protocol information
 

src/developer/debug/zxdb/README.md

+# Zxdb
+
+This is the code for the frontend of the Fuchsia debugger. This frontend runs
+on the developer's host computer (Linux or Mac) and talks via IPC (code in
+[../ipc](../ipc)) to the debug agent (code in [../debug_agent](../debug_agent))
+running on the Fucsia target.
+
+### User documentation
+
+Please see the debugger [setup](../../../../garnet/docs/debugger.md) and
+[usage](../../../../garnet/docs/debugger_usage.md) documentation.
+
+### Subdirectories
+
+In order from the lowest conceptual level to the highest. Dependencies always
+point "up" in this list.
+
+  * `common`: Lower-level utilities used by multiple other layers. Can not
+    depend on any other part of the debugger.
+
+  * `symbols`: The symbol library. This wraps LLVM's DWARF parser and provides
+    symbol indexing, an object model, and helper utilities for dealing with
+    symbols.
+
+  * `expr`: The expression evaluation library. This provides a parser for
+    C++-like expressions and an execution environment for these expressions
+    using the symbol library.
+
+  * `client`: Conceptually this is a library for writing a debugger UI. It
+    provides functions for the lower-level commands like "step" and "next" and
+    an object model for dealing with processes, threads, etc. But this library
+    provides no user-interface.
+
+  * `console`: Frontend for the client that provides a console UI on Linux and
+    Mac.