update readme with CCSDS info

Author: Michal-Pogorzelec

README.md

@@ -1,5 +1,5 @@
 # EmbeddedSpacePacket
-Minimum SpacePacket implementation in C for embedded applications
+Minimum CCSDS SpacePacket implementation in C for embedded applications.
 
 Contents
 - tiny header and implementation in `include/` and `src/`
@@ -27,3 +27,26 @@ Notes
   in the header stores `payload_len - 1` when serializing and the parser reconstructs
   `payload_len = header_length + 1`.
 
+CCSDS Space Packet (brief)
+- **Primary header (6 bytes):** bytes 0-1 hold version (3 bits), packet type (1 bit),
+	secondary header flag (1 bit) and APID (11 bits). Bytes 2-3 are the sequence control
+	field: segmentation/sequence flags (2 bits) and packet sequence count (14 bits). Bytes
+	4-5 are the Packet Length field (16 bits) which encodes the length of the Packet Data
+	Field minus 1 (i.e., PacketLength = N-1 where N is data-field octets).
+- **Packet Data Field:** follows the primary header and contains an optional secondary
+	header (if the secondary header flag is set) followed by user/application data.
+- **Secondary header:** optional, format and semantics are application-dependent (time
+	tags, service headers, etc.). The presence is indicated by the secondary header flag.
+- **Segmentation/Sequence Flags:** indicate if the packet is standalone (unsegmented),
+	first, continuation or last segment when a logical packet is split over multiple
+	sequence counts.
+- **APID (Application Process ID):** identifies the source/consumer application or
+	logical stream of packets.
+- **Error control:** CCSDS Space Packet by itself does not mandate a particular CRC in
+	the primary header; error control is usually provided in lower or higher layers or
+	via application-specific fields. This repository adds an optional CRC covering the
+	secondary header and payload as an example.
+
+
+Links:
+- https://ccsds.org/Pubs/133x0b2e2.pdf (CCSDS Space Packet Standard)