Add a readme for the symbol server backend
Add a readme for the symbol server backend

Bug: chromium:1077759
Change-Id: Iee07719b03565acc25be90af3fb46fabe429ae09
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2181214
Commit-Queue: Philip Pfaffe <639a645feae3653b7aaefa0387217d375f2f82ff@chromium.org>
Reviewed-by: Benedikt Meurer <024a1e0af501338d11c3b837c6193d11c140d792@chromium.org>
Reviewed-by: Eric Leese <b4befed3d60c01ea229cdb14329781bec1b07e7f@chromium.org>
Reviewed-by: Kim-Anh Tran <a72f38a79b5fac6811022b8a7fb84830d856c9c9@chromium.org>

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