redfish-doc: Add details on EventLog implementations

Was struggling a bit reviewing the code and options in the LogServices
area so wrote this up quickly. The main goal being to give a brief
overview and link to some documentation to help with reviewing the
options.

Change-Id: I337490157ffdb188be812031311694f6cb83b8fa
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/Redfish.md b/Redfish.md
index 1d4992c..74322c9 100644
--- a/Redfish.md
+++ b/Redfish.md
@@ -1,5 +1,10 @@
 # Redfish #
 
+bmcweb provides an implementation of the [Redfish][1] API.  This document
+details the Redfish schemas supported by bmcweb. This document also discusses
+some of the details of that implementation and different implementations
+available for certain areas.
+
 ## Redfish Schema
 
 The redfish implementation shall pass the [Redfish Service
@@ -260,6 +265,32 @@
 - MaxIPv6StaticAddresses
 
 #### /redfish/v1/Managers/BMC/LogServices
+
+The [LogService][2] resource provides properties for monitoring and configuring
+events for the service or resource to which it is associated.
+
+Within bmcweb, the LogService object resides under the System resource. It
+tracks all events for the system.
+
+The LogService supports multiple log entry types. bmcweb has support for
+the `Event` type. This is the new Redfish-defined type.
+
+bmcweb supports two different implementations of the
+`LogService/EventLog/Entries` URI.
+
+The default implementation uses rsyslog to write Redfish events from the journal
+to the persistent /var/log/ filesystem. The bmcweb software then looks for these
+files in /var/log/ and returns the appropriate Redfish EventLog Entries for
+these. More details on adding events can be found [here][3]
+
+The other implementation of EventLog Entries can be enabled by compiling bmcweb
+with the `-DBMCWEB_ENABLE_REDFISH_DBUS_LOG_ENTRIES=ON` option. This will cause
+bmcweb to look to [phosphor-logging][4] for any D-Bus log entries. These will
+then be translated to Redfish EventLog Entries.
+
+These two implementations do not work together, so choosing one will disable
+the other.
+
 ##### LogServiceCollection
 - Members
 - Members@odata.count
@@ -460,3 +491,8 @@
 #### /redfish/v1/UpdateService/SoftwareInventory/{MemberName}
 ##### SoftwareInventory
 - Version
+
+[1]: https://www.dmtf.org/standards/redfish
+[2]: https://redfish.dmtf.org/schemas/v1/LogService.json
+[3]: https://github.com/openbmc/docs/blob/master/redfish-logging-in-bmcweb.md
+[4]: https://github.com/openbmc/phosphor-logging