docs: add notes on extending intel_fw

docs/SUMMARY.md

@@ -1,5 +1,6 @@
 - [`intel_fw`](intro.md)
     - [Architecture and Design](architecture.md)
+    - [Extend with new variants and features](extend.md)
 - [Intel platforms](platforms.md)
     - [Knowledge on Firmware Images](knowledge.md)
     - [Obtaining firmware images](images.md)

docs/extend.md

@@ -0,0 +1,39 @@
+# Extend with new variants and features
+
+The following is a brief list of guiding principles to extend `intel_fw`.
+As a library, it should to adhere to those as requirements in order to provide
+guarantees to applications working with it. Generally, it should follow the
+[Rust API guidelines](https://rust-lang.github.io/api-guidelines/about.html).
+
+## Do not panic!
+
+Parsing firmware means looking for and acting on offsets and sizes frequently,
+and they always need to be checked to stay within the bounds of the given data.
+Never `unwrap()`, which means an _intentional panic_, in case something cannot
+be found, read, or recognized. Instead, return instances of `Self` for structs,
+wrapped in a `Result<Self, CustomError>` or possibly `Option<Self>`.
+
+## Continuous parsing
+
+There are circumstances under which a parser encounters an issue. It can be
+that offsets do not make sense, magic bytes (struct markers) are not as they
+were expected, or new variants are found with samples not encountered before.
+In those situations, it is desirable to follow a best-effort strategy. I.e.,
+when there is still remaining data that could be parsed, keep going.
+
+## Let apps provide feedback
+
+In other words, **let the consuming application take care** of taking the result
+apart. Nested structures mean that whenever a node in a graph turns into an
+`Error` or `None`, other nodes beside it may still provide useful information.
+That information helps the user of an app to understand the context and possibly
+report what they are facing or look into the issue themselves.
+
+## Errors and context
+
+In order to make sense of an error, context is important. During development, it
+can be helpful to print out information right within a specific parser. However,
+a final app is typically not run in development mode, but as a release. In that
+moment, semantic errors will help to identify possible problems. Include offsets
+and sizes (or `Range`s) for the application to tell exactly where the problem
+is, and it can choose to e.g. dump a contextual hex view on the data.