Add more detail on the context/rationale that should be included when contributing (#2432)
Add more detail on the context/rationale that should be included when contributing (#2432)

Follow-up to https://github.com/matrix-org/matrix-js-sdk/pull/1933

Spawning from various recent documents and comments:

 - https://github.com/vector-im/element-meta/wiki/Review-process
 - https://github.com/matrix-org/synapse/pull/12846#discussion_r887270734
 - https://gitlab.matrix.org/new-vector/internal/-/wikis/Backend/Reviews
matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk