Initial documentation of fprime libraries (#2665)

* Initial documentation of fprime libraries

* Update Library How-To Guide

* Finished first-pass on library documentation

* sp

* Add link to how-to contents

* Review recommendations - part 1

* Review recommendations - part 2

docs/HowTo/develop-fprime-libraries.md

@@ -0,0 +1,85 @@
+# How-To: Develop an F´ Library
+
+This guide will walk you through the structure and best practices in developing an F´ library.  F´ libraries are used to share modules (components, ports, topologies, etc.), toolchains, and platforms between F´ projects.  These libraries help with code reuse between projects by enabling direct sharing of F´ software.
+
+*Contents:*
+1. [F´ Library Structure](#flibrary-structure)
+2. [Required: `library.cmake`](#required-librarycmake)
+3. [Optional: Toolchain Folder and Toolchain Files](#optional-toolchain-folder-and-toolchain-files)
+4. [Optional: Platform Folder and Platform Files](#optional-platform-folder-and-platform-files)
+5. [Optional: F´ Module Directories](#optional-fmodule-directories)
+
+## F´ Library Structure
+
+In this section, you will learn about the expected structure of an F´ library. An F´ library must have a `library.cmake` and may have any of the following items:
+
+1. Module Directories and Modules (Components, Ports, Topologies, etc.)
+2. `cmake/toolchain` Folder and Toolchain Files
+3. `cmake/platform` Folder and Platform Files
+
+That means that a complete F´ library might look like the following:
+
+```bash
+my-library/
+├── cmake
+│   ├── platform
+│   └── toolchain
+├── MyLibrary
+│   ├── Components
+│   │   └── MyComponent
+│   │       └── ...
+│   └── MyDeployment
+│       └── ...
+└── library.cmake
+```
+
+> Module directories shall be namespaced (i.e. `MyLibrary` above) to ensure that modules do not collide with F´ and other libraries. Placing container directories (e.g. `Components`, `Svc`, `Drv`, etc.) directly at the root of the repository is *strongly* forbidden.
+
+You will see how to develop each of these parts of the library in the following sections.
+
+##  Required: `library.cmake`
+
+The `library.cmake` file is the only required file for an F´ library.  It consists of a series of `add_fprime_subdirectory` calls that include the various modules that are part of the library.  For libraries that just contain toolchains and platforms, this file may be empty. 
+
+**Sample `library.cmake`**
+```cmake
+####
+# library.cmake for MyLibrary
+####
+add_fprime_subdirectory("${CURRENT_CMAKE_LIST_DIR}/MyLibrary/Components/MyComponent")
+add_fprime_subdirectory("${CURRENT_CMAKE_LIST_DIR}/MyLibrary/MyTopology")
+```
+
+`library.cmake` should exist at the root of the library's directory structure.
+
+
+## Optional: F´ Module Directories
+
+F´ libraries share F´ code through modules. These module directories are created by `fprime-util new --component` or by hand and may include any F´ code (Components, Ports, Topologies, Data Types, etc.).  These types are made available to users of the library by ensuring that they are added to the `library.cmake` file.  The only restriction is that these component should be placed under a namespacing directory for the library (e.g. `MyLibrary`) to avoid collisions with other libraries and F´ code.
+
+Developing component, ports, topologies, etc. are the subject of our [tutorials](../Tutorials/README.md).
+
+
+## Optional: Toolchain Folder and Toolchain Files
+
+F´ libraries often need to supply new toolchain integrations with F´. These libraries must define a `cmake/toolchain` folder and place toolchain files within that folder.  Any file ending in `.cmake` and placed within the `cmake/toolchain` folder will be available as a toolchain via `fprime-util generate <toolchain>`.
+
+The example toolchain above (`cmake/toolchain/my-toolchain`) would be found as `my-toolchain` in `fprime-util`.
+
+```bash
+fprime-util generate my-toolchain
+```
+
+The `cmake/toolchain` folder may contain any number of toolchains and must be placed in the root of the library's directory structure.
+
+## Optional: Platform Folder and Platform Files
+
+In a similar manner to toolchains, platforms may be provided in the `cmake/platform` folder. Any file ending in `.cmake` and placed within the `cmake/platform` folder will be available to F´  to be loaded as a platform. Commonly, this means that a toolchain would include the line `set(FPRIME_PLATFORM <platform>)`.
+
+The example platform would thus need to include `set(FPRIME_PLATFORM "my-platform")`.
+
+The `cmake/platform` folder may contain any number of platform files and must be placed in the root of the library's directory structure.
+
+## Conclusion
+
+You should now understand the structure of an F´  library an know how to assemble one.

docs/HowTo/general.md

@@ -6,4 +6,5 @@ This section of the documentation captures how-to guides for various F´ develop
 | Table of Contents                          |
 |--------------------------------------------|
 | [F´ GDS Plugins](./develop-gds-plugins.md) |
+| [F´ Libraries](./develop-fprime-libraries.md) |
 

docs/_includes/toc.md

@@ -103,4 +103,5 @@
 <h4><a href="/fprime/Design/general.html">How-To Guides</a></h4>
   <ul>
     <li><a href="/fprime/HowTo/develop-gds-plugins.html">F´ GDS Plugin Development</a></li>
+    <li><a href="/fprime/HowTo/develop-fprime-libraries.html">F´ Library Development</a></li>
   </ul>