feature/vcgg: Add vgcc program readme.

Issue ufs-community#136

sorc/vcgg.fd/readme

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+
+    This program generates hybrid coordinate interface profiles
+    from a few given parameters. The hybrid coordinate is intended to start
+    out at the bottom in pure sigma and end up at the top in pure pressure,
+    with a smooth transition in between. The pressure thickness is close to
+    quadratic in pressure, with maximum thicknesses in the middle of the domain.
+    The coordinate pressure will have continuous second derivatives in level.
+
+    The hybrid coordinate is returned in terms of vectors AK and BK, where
+    the interface pressure is defined as A+B*ps where ps is surface pressure
+    Thus A=0 in regions of pure sigma and B=0 in regions of pure sigma.
+    At the bottom, A(0)=0 and B(0)=1 so that surface pressure is the bottom
+    boundary condition, while at the top, A(levs)=ptop and B(levs)=0 so that
+    the constant top pressure (which can be zero) is the top boundary condition.
+
+    Input argument list:
+      levs     integer number of levels 
+      lupp     integer number of levels below pupp
+      pbot     real nominal surface pressure (Pa) 
+      psig     real nominal pressure where coordinate changes
+               from pure sigma (Pa) 
+      ppre     real nominal pressure where coordinate changes
+               to pure pressure (Pa) 
+      pupp     real nominal pressure where coordinate changes
+               to upper atmospheric profile (Pa)
+      ptop     real pressure at top (Pa)
+      dpbot    real coordinate thickness at bottom (Pa) 
+      dpsig    real thickness of zone within which coordinate changes
+               to pure sigma (Pa) 
+      dppre    real thickness of zone within which coordinate changes
+               to pure pressure (Pa) 
+      dpupp    real coordinate thickness at pupp (Pa) 
+      dptop    real coordinate thickness at top (Pa) 
+
+
+    Graphical description of parameters and zones:
+      ptop  ---  -----  ----------------------
+            ...  dptop
+            ---         zone U (upper atmos)
+            ...
+      pupp  ---  -----  ----------------------
+            ...  dpupp
+            ---  -----
+            ...         zone P (pure pressure)
+            ---
+            ...
+      ppre  ---  -----  ----------------------
+            ...
+            ---  dppre  zone T1 (transition 1)
+            ...
+            ---  -----  ----------------------
+            ...
+            ---
+            ...         zone T2 (transition 2)
+            ---
+            ...
+            ---  -----  ----------------------
+            ...
+            ---  dpsig  zone T3 (transition 3)
+            ...
+      psig  ---  -----  ----------------------
+            ...
+            ---  -----  zone S (pure sigma)
+            ...  dpbot
+      pbot  ---  -----  ----------------------
+
+    For details on the procedure, see the program prolog.