Add example of variable length field calculations to documentation

docs/source/xtce_variable_length_packets.md

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+# XTCE Documentation on Variable Length Packets
+
+The intent of this documentation is to describe the process of defining variable length packets in XTCE.
+
+## Explanation
+The PKT_LEN field is the length of the packet in bytes. 
+PKT_LEN a) counts from zero and b) does not include the header bits. 
+Hence, the length of the parameter in bits (PARAM_LEN) may be calculated by the following equation:
+
+PARAM_LEN = 8*(PKT_LEN + 1) - FIXED_LENGTH_DATA
+
+where FIXED_LENGTH_DATA is the sum of the length of each data field in bits. 
+
+An example of this is shown below.
+
+## Example
+**Packet Definition:**  
+"VERSION" : 3 bits  
+"TYPE" : 1 bits  
+"SEC_HDR_FLG" : 1 bits  
+"PKT_APID" : 11 bits  
+"SEQ_FLGS" : 2 bits  
+"SRC_SEQ_CTR" : 14 bits  
+"PKT_LEN" : 16 bits  
+"SHCOARSE" : 32 bits  
+"SID" : 8 bits  
+"SPIN" : 8 bits  
+"ABORTFLAG" : 1 bits  
+"STARTDELAY" : 15 bits  
+"COUNT" : 8 bits  
+"EVENTDATA": variable length
+
+FIXED_LENGTH_DATA = 32+8+8+1+15+8  
+
+Therefore:  
+EVENTDATA = 8 * (PKT_LEN + 1) - 72
+
+This equation can be implemented in XTCE as follows:
+
+```xml
+<xtce:BinaryParameterType name="eventdata">
+    <xtce:UnitSet/>
+    <xtce:BinaryDataEncoding>
+        <xtce:SizeInBits>
+            <xtce:DynamicValue>
+                <xtce:ParameterInstanceRef parameterRef="PKT_LEN"/>
+                <xtce:LinearAdjustment intercept="-64" slope="8"/>
+            </xtce:DynamicValue>
+        </xtce:SizeInBits>
+    </xtce:BinaryDataEncoding>
+</xtce:BinaryParameterType>
+```