Added README for corrections

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+Corrections for PWGLF/FORWARD Analysis
+======================================
+
+This directory contain 2 files:
+
+* fmd_corrections.root: Database of corrections used by the FMD part
+  of PWGLF/FORWARD
+* spd_corrections.root: Database of corrections used by the SPD part
+  of PWGLF/FORWARD
+
+General Remarks
+---------------
+
+The code to store and retrieve lives in AliCorrectionManagerBase with
+specific sub-classes AliFMDCorrectionManager and
+AliSPDCorrectionManager. The base class - and it's contained classes -
+has all the functionality for retrieving and storing the corrections
+in a TTree (one tree for each correction) while the sub-classes merely
+registers which corrections exists and provide convenience functions.
+
+The data is stored in TTrees - one for each correction. Each entry in
+the trees correspond to a single instance of a correction. The
+correction object itself is wrapped in a AliOADBForward::Entry
+object, which provide meta data such as
+
+* run number
+* collision system (1: pp, 2: PbPb, 3: pPb)
+* collision energy in GeV
+* L3 magnetic field in kG
+* whether the correction can be used for satellite analysis
+* whether the correction is for simulated or real data
+* date of submission
+* author of the submission.
+
+The code to handle these trees and the entries lives in
+AliOADBForward.
+
+Entries in the trees are indexed by one or more of the above mentioned
+meta fields.  Which fields are used for a given correction is defined
+by the sub-classes of AliCorrectionManagerBase.
+
+In general, a query will only succeed if all fields of the query
+matches an entry.  If this returns more than one result, the most
+recent one is used.  However, there are some exceptions to this.
+
+Run Number Selection
+--------------------
+In general, one cannot expect to get an entry with the exact run
+number queried.  AliOADBForward therefor defines a set of policies for
+how to match a given run number
+
+* EXACT - run number must match exactly
+* NEAR - the run number of the selected entries must not be too far
+  from the run number given in the query.
+* OLDER - entries with a run number less than or equal to the queried
+  run number are selected
+* NEWER - entries with a run number greater than or equal to the
+  queried run number are selected
+
+Which mode to use for a given correction is defined on creation of the
+database, but can be overridden later on.
+
+If no entry is found with the specified run number and mode, then the
+correction may allow for a fall-back option.  These are entries with
+run number equal to 0.  In this case, all other active meta fields
+must match.
+
+Fall-back Mode
+--------------
+
+If a general fall-back mode is enabled, then if no entry was selected
+using the full set of fields, then we can ignore the collision
+energy.  If this still does not return anything, we can ignore all
+fields, which means we will select the most recent entry -
+irrespective of other meta fields.
+
+
+
+<!--  Local Variables: -->
+<!--   mode: markdown -->
+<!--  End --> 
+<!--  LocalWords:  AliCorrectionManagerBase AliFMDCorrectionManager -->
+<!--  LocalWords:  AliSPDCorrectionManager TTree TTrees PbPb pPb GeV
+<!--  LocalWords:  AliOADBForward kG
+ -->