fix(docs): add browser support section
fix(docs): add browser support section

Fixes #2413

[skip ci]
vladikoff/fxa-content-server,vladikoff/fxa-content-server,mozilla/fxa-content-server,mozilla/fxa-content-server,swatilk/fxa-content-server,swatilk/fxa-content-server,vladikoff/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,swatilk/fxa-content-server