Document when one should use NeuroML for creating new ComponentTypes, and when one should use LEMS

[sanjayankur31]

• When creating new channel ComponentTypes, one generally uses HH etc components which are defined in the NeuroML standard. So, new channels can be extended using NeuroML itself. i.e., they use <NeuroML> and will be valid against the NeuroML schema. The NeuroML schema includes a subset of LEMS elements to allow this: https://github.com/NeuroML/NeuroML2/blob/development/Schemas/NeuroML2/NeuroML_v2.3.xsd#L312
• When creating new synapse ComponentTypes, the LEMS bits included in NeuroML may not be sufficient. One often needs OnStart and other elements that are not included in the NeuroML schema. So, one has to use LEMS to define these. I.e., these files will use <LEMS> as the root element, and they will validate against the LEMS schema (use something like xmllint to validate the file).

This needs to be documented, with examples, so that it's clear to users when they should use what.