fix: Add more detail on licensing

docs/dev-guide/licensing.md

@@ -1,38 +1,86 @@
 # Licensing
 
-With so many options for licensing code, the topic can be quite confusing.
+With so many options for licensing code, the topic can be quite
+confusing. Many projects like Boost, Eclipse, Apache, etc.. have their
+own license, often based on the
+[GPL](https://en.wikipedia.org/wiki/GNU_General_Public_License), which
+only apply to that project, so aren't appropriate for use at
+HOT. There are two primary types of licenses, permissive and
+non-permissive. Non permissive licenses may appear to be open source,
+but often it's a few clauses in the licenses that make them unable to
+be used in other open source projects.
 
-Simply, at HOT we use two types of license:
+HOT believes strongly in the freedoms embodied on the [free
+software](https://en.wikipedia.org/wiki/Free_software) culture, while
+following the collaborative development processes common for open
+source software projects.
 
-## All code is AGPLv3
+All of the software at HOT uses permissive licenses, commonly the
+GPLv3 for compiled code, the AGPLv3 for code shared over a network (a
+website), and some older code using the BSD license or pubic domain.
 
-- This is a strong 'copyleft' licence, meaning that anyone using a modified
-  version of our open-source code, **must make their code open too**.
-- The main purpose of this license is to prevent commercial exploitation of
-  open code, making any modifications open so the entire community can benefit.
+## The GPLv3 and AGPLv3
+
+- These are strong 'copyleft' licences, meaning that anyone
+modifiying the software is required to make those changes openly
+available. This is commonly called the derivative work clause. Any
+derivative software must be contributes it back to the original
+software project with no restrictions.
+
+- The main purpose of this license is to prevent commercial
+  exploitation of open code, making any modifications open so the
+  entire community can benefit.
 
 ## Other Content is CC-BY
 
-- Creative content, such as translations and designs are copyrighted different.
+- Creative content, such as translations and designs are copyrighted
+  different.
 - As with AGPL for code, this license is 'copyleft'
-- It allows others modify and built upon our own, even for commercial purposes,
-  as long as they **credit us** and
+- It allows others modify and built upon our own, even for commercial
+  purposes, as long as they **credit us** and
   **license their work under the same terms as CC-BY**.
 
 ## What Licenses Can We Use
 
 - We use many underlying packages and tools within our software.
-- Any contributor must be aware of licensing before adding any additional
-  external code to our software.
+- Any contributor must be aware of licensing before adding any
+  additional external code to our software.
+
+### Compatible Licenses
+
+This is a short list of copyleft compatible licenses. For more detail,
+refer to the Free Software Foundation (FSF) page on [compatible and
+incompatible
+licenses](https://www.gnu.org/licenses/license-list.en.html). This in
+far from an exhaustive list, only the most common ones are listed
+here.
+
+* Public Domain
+* Mozilla Public License (MPL) version 2.0
+* Apache License 2.0
+* Modified BSD license
+* Intel Open Source License
+* FreeBSD license
 
-### Can Use
+### Incompatible Licenses
 
-- xxx
-- xxx
-- xxx
+The Open Source Initiative also has a list of [approved open source
+licenses](https://opensource.org/licenses). Not all of these are
+compatible with copylefted software, so not appropriate for code at
+HOT even though they are considered premissive licenses.
 
-### Can't Use
+Here is a short list of common non-permissive licenses, and can't be
+used for any code in HOT software projects.
 
-- xxx
-- xxx
-- xxx
+* Mozilla Public License (MPL) version 1.1
+* Apache License, Version 1.0 and 1.1
+* Apple Public Source License (APSL), version 2
+* Microsoft Public License (Ms-PL)
+* NASA Open Source Agreement
+* Original BSD license
+* Common Development and Distribution License (CDDL), version 1.0
+* Common Public Attribution License 1.0 (CPAL)
+* Common Public License Version 1.0
+* European Union Public License (EUPL) version 1.1 and 1.2
+* IBM Public License, Version 1.0
+* Open Software License, all versions through 3.0