DOC: update documentation.

Add documentation covering:
  - developer information about architecture
  - project structure and building
  - project overview and service descriptions
  - the portability model for supporting hardware

Signed-off-by: Julian Hall <julian.hall@arm.com>
Co-Authored-By: Gyorgy Szing <gyorgy.szing@gmail.com>
Change-Id: I8bf9c01a66350719d82a7ca2bc1c78a8ab17978d
diff --git a/docs/uml/style-component.puml b/docs/uml/style-component.puml
new file mode 100644
index 0000000..9400e93
--- /dev/null
+++ b/docs/uml/style-component.puml
@@ -0,0 +1,28 @@
+'-------------------------------------------------------------------------------
+' Copyright (c) 2020-2021, Arm Limited and Contributors. All rights reserved.
+'
+' SPDX-License-Identifier: BSD-3-Clause
+'
+'-------------------------------------------------------------------------------
+
+@startuml
+!include colors.puml
+
+'Comonent diagram definitions
+!define NS_COMPONENT component COLOR_GREY2
+!define NS_IFC interface COLOR_GREY2
+!define S_COMPONENT component COLOR_BLUE1
+!define S_IFC interface COLOR_BLUE1
+
+skinparam component {
+	ArrowColor black
+	BorderColor black
+}
+
+skinparam interface {
+	BorderColor black
+}
+
+skinparam BackgroundColor COLOR_ARM_BLUE
+
+@enduml