Added README file for parameters.

Filled in information about drift chamber parameters. It can serve
as a template for other good people :)

PARAM/README.md

+# Notes
+
+## Naming
+
+* `X` stands for spectrometer name:
+  * `h` for HMS
+  * `p` for SHMS
+  * `s` for SOS
+* `Y` stands for plane name.
+* `N` stands for sequential number
+
+## Constants
+
+* `raddeg = 3.14159265/180`
+  * Conversion from degrees to radians.
+  * real
+  * `DBASE/general.param`
+
+
+# Tracking
+
+* `Xsel_using_scin = 0`
+  * Use scintillator for track selection.
+  * {0, 1} bool
+  * `THcHallCSpectrometer::fSelUsingScin`
+
+# Drift chambers
+
+## `Xdc.param`
+
+* `Xdc_sigma`
+  * Sigma of wire chamber resolution for each plane.
+  * [double]
+  * `THcDC::fSigma`
+* `Xdc_tdc_min_win`
+  * TDC window lower limit for each plane.
+  * [int]
+  * `THcDC::fTdcWinMin`
+* `Xdc_tdc_max_win`
+  * TDC window higher limit for each plane.
+  * [int]
+  * `THcDC::fTdcWinMax`
+* `Xdc_tdc_time_per_channel`
+  * Conversion factor from TDC clicks to ns.
+  * double
+  * `THcDC::fNSperChan`
+* `Xdc_plane_time_zero`
+  * Zero time correction for each plane in ns that is added to TDC time.
+  * [double]
+  * `THcDC::fPlaneTimeZero`
+* `Xdc_wire_velocity`
+  * Wire velocity.
+  * double
+  * `THcDC::fWireVelocity`
+* `Xdc_central_time`
+  * Some sort of central time for each plane. Used for hit time correction.
+  * [int]
+  * `THcDC::fCentralTime`
+
+## `Xdc.pos`
+
+* `Xdc_num_planes`
+  * Total number of drift chamber planes in detector.
+  * int
+  * `THcDC::fNPlanes`
+* `Xdc_num_chambers`
+  * Number of drift chambers in detector.
+  * int
+  * `THcDC::fNChambers`
+* `Xdc_zpos`
+  * z position in cm for each plane measured from focal plane.
+  * [double]
+  * `THcDC::fZPos`
+* `Xdc_alpha_angle`
+  * Angle alpha of wires for each plane in radians. Basically, angle of wires
+    in x-y plane.
+  * [double]
+  * `THcDC::fAlphaAngle`
+* `Xdc_beta_angle`
+  * Beta angle of wires for each plane in radians.
+  * [double]
+  * `THcDC::fBetaAngle`
+* `Xdc_gamma_angle`
+  * Angle gamma of wires for each plane in radians
+  * [double]
+  * `THcDC::fGammaAngle`
+* `Xdc_pitch`
+  * Pitch between wires for each plane.
+  * [double]
+  * `THcDC::fPitch`
+* `Xdc_nrwire`
+  * Number of wires in each plane.
+  * [int]
+  * `THcDC::fNWires`
+* `Xdc_xcenter`
+  * x position of center of each wire chamber.
+  * [double]
+  * `THcDC::fXCenter`
+* `Xdc_ycenter`
+  * y position of center of each wire chamber.
+  * [double]
+  * `THcDC::fYCenter`
+* `Xdc_central_wire`
+  * Wire number of center of wire chamber for each plane. Basically, this is
+    number of imaginary wire, that would cross center of plane. Wire numbers
+    start with 1.
+  * [double]
+  * `THcDC::fCentralWire`
+* `Xdc_chamber_planes`
+  * Number chamber that each plane belongs to. Chamber number starts with 1.
+  * [int]
+  * `THcDC::fNChamber`
+* `Xdc_wire_counting`
+  * Order in which wires are counted in each plane.
+    * `Xdc_wire_counting == 0 : center = (wire - Xdc_central_wire) * pitch`
+    * `Xdc_wire_counting == 1 : center = (Xdc_nrwire + 1 - wire + Xdc_central_wire) * pitch`
+  * [{0, 1}]
+  * `THcDC::fWireOrder`
+* `Xdc_drifttime_sign`
+  * Used for velocity correction in each plane. The velocity correction is the
+    distance from the center of the wire divided by the velocity of propagation
+    times Xdc_drifttime_sign(plane). +/-1 for disc. card at +/- coord. (i.e.
+    top = -x direction, so top readout is +1)
+  * [{-1, 1}]
+  * `THcDC::fDriftTimeSign`
+
+## `driftmap.param`
+
+Contains lookup table for drift time.
+
+* `Xdriftbins`
+  * Number of bins in table.
+  * int
+  * `THcDriftChamberPlane::ReadDatabase::NumDriftMapBins`
+* `Xdrift1stbin`
+  * Time of first bin in ns.
+  * double
+  * `THcDriftChamberPlane::ReadDatabase::DriftMapFirstBin`
+* `Xdriftbinsz`
+  * Bin size in ns.
+  * double
+  * `THcDriftChamberPlane::ReadDatabase::DriftMapBinSize`
+* `XwcYfract`
+  * Drift time lookup table entries for plane Y.
+  * [double]
+  * `THcDriftChamberPlane::ReadDatabase::DriftMap`