Intermediate Representation docs (#6685)

* Intermediate Representation docs

* minor rewording

* addressing comments

* move IR to bottom

Co-authored-by: Casper Neo <cneo@google.com>

docs/source/IntermediateRepresentation.md

@@ -0,0 +1,32 @@
+# Flatbuffers Intermediate Representation {#intermediate_representation}
+
+We use [reflection.fbs](https://github.com/google/flatbuffers/blob/master/reflection/reflection.fbs)
+as our intermediate representation. `flatc` parses `.fbs` files, checks them for
+errors and stores the resulting data in this IR, outputting `.bfbs` files.
+Since this IR is a Flatbuffer, you can load and use it at runtime for runtime
+reflection purposes.
+
+There are some quirks: 
+- Tables and Structs are serialized as `Object`s.
+- Unions and Enums are serialized as `Enum`s.
+- It is the responsibility of the code generator to check the `advanced_features`
+  field of `Schema`. These mark the presence of new, backwards incompatible,
+  schema features. Code generators must error if generating a schema with
+  unrecognized advanced features.
+
+
+## Invocation 
+You can invoke it like so
+```{.sh}
+flatc -b --schema ${your_fbs_files}
+```
+This generates `.bfbs` (binary flatbuffer schema) files.
+
+Some information is not included by default. See the `--bfbs-filenames` and
+`--bfbs-comments` flags. These may be necessary for code-generators, so they can
+add documentation and maybe name generated files (depending on the generator).
+
+
+TODO(cneo): Flags to output bfbs as flexbuffers or json.
+
+TODO(cneo): Tutorial for building a flatc plugin.