util/spd_tools: Document adding support for a new memory technology

Add documentation describing how to add support for a new memory
technology to spd_tools:
- Add a section to the README.
- Document the memTech interface in spd_gen.go.

BUG=b:191776301
TEST=None

Signed-off-by: Reka Norman <rekanorman@google.com>
Change-Id: Ie710c1c686ddf5288db35cf43e5f1ac9b1974305
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59005
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/spd_tools/src/spd_gen/spd_gen.go

@@ -28,10 +28,37 @@ type memPart struct {
 }
 
 type memTech interface {
+	/*
+	 * Returns the set -> platform mapping for the memory technology. Platforms with the
+	 * same SPD requirements should be grouped together into a single set.
+	 */
 	getSetMap() map[int][]int
+
+	/*
+	 * Takes the name and attributes of a part, as read from the memory_parts JSON file.
+	 * Validates the attributes, returning an error if any attribute has an invalid value.
+	 * Stores the name and attributes internally to be used later.
+	 */
 	addNewPart(string, interface{}) error
+
+	/*
+	 * Takes the name of a part and a set number.
+	 * Retrieves the part's attributes which were stored by addNewPart(). Updates them by
+	 * setting any optional attributes which weren't specified in the JSON file to their
+	 * default values.
+	 * Returns these updated attributes.
+	 */
 	getSPDAttribs(string, int) (interface{}, error)
+
+	/*
+	 * Returns the size of an SPD file for this memory technology.
+	 */
 	getSPDLen() int
+
+	/*
+	 * Takes an SPD byte index and the attributes of a part.
+	 * Returns the value which that SPD byte should be set to based on the attributes.
+	 */
 	getSPDByte(int, interface{}) byte
 }