Add services toplevel & README

TBR=jam@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1876543002

Cr-Commit-Position: refs/heads/master@{#386284}

services/README.md

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+Chrome Platform Services
+====
+
+### Overview
+
+This directory contains Chrome Platform Services. If you think of Chrome as a
+"portable OS," Chrome Platform Services can be thought of as that OS' "system
+services".
+
+Roughly each subdirectory here corresponds to a service that:
+
+  * is a client of `//services/shell` with its own unique Identity.
+  * could logically run a standalone process for security/performance isolation
+    benefits depending on the constraints of the host OS.
+
+### Service Directory Structure
+
+Individual services are structured like so:
+
+    //services/foo/                   <-- Implementation code, may have subdirs.
+                  /public/    
+                         /cpp/        <-- C++ client libraries (optional)
+                         /interfaces/ <-- Mojom interfaces
+
+### Dependencies
+
+Code within `//services` may only depend on each other via each other's
+`/public/` directories, i.e. implementation code cannot be shared directly.
+
+Service code should also take care to tightly limit the dependencies on static
+libraries from outside of `//services`. Dependencies to large platform
+layers like `//content`, `//chrome` or `//third_party/WebKit` must be avoided.
+
+### Physical Packaging
+
+Note that while it may be possible to build a discrete physical package (DSO)
+for each service, products consuming these services may package them
+differently, e.g. by combining them into a single package.