Update README to make it clearer what fgcmcal does within the stack.

README.md

@@ -1,19 +1,25 @@
-fgcmcal: Forward Global Calibration Method (FGCM) for LSST
+fgcmcal: Global Photometric Calibration in LSST with FGCM
 =========================================================
 
-This is LSST stack code to interface with the FGCM package.  See
-http://adsabs.harvard.edu/abs/2018AJ....155...41B for the paper describing the
-method, and https://github.com/erykoff/fgcm (as forked in
-https://github.com/lsst/fgcm ) for the Python implementation that is
-used by this LSST package.
-
-FGCM performs global survey calibration with forward modeling of the atmosphere
-and instrument, including chromatic corrections.  Currently, this LSST package
-will only run on HSC data within the stack, but this is being actively worked
-on.  In general, the FGCM code should be compatible with any survey with the
-following requirements:
-
-* Exposure/CCD based observations
+This is the LSST stack code to interface with the Forward Global Calibration
+Method (FGCM) third-party package to perform global photometric survey
+calibration.  Please see [Burke, Rykoff, et
+al. 2018](http://adsabs.harvard.edu/abs/2018AJ....155...41B) for the paper
+describing the method, and https://github.com/lsst/fgcm/tree/lsst-dev (the LSST
+fork of https://github.com/erykoff/fgcm) for the Python implementation that is
+used by this `fgcmcal` package.
+
+FGCM performs a global photometric calibration, starting with instrumental
+fluxes and producing top-of-the-atmosphere standard fluxes by forward modeling
+the atmosphere and instrument, including chromatic corrections.  Overall
+absolute calibration is reduced to the computation of one offset per band,
+which `fgcmcal` can optionally compute based on a reference catalog.
+
+Currently, the `fgcmcal` package will only run on HSC data within the stack,
+but this is being actively worked on.  In general, the FGCM code should be
+compatible with any survey with the following requirements:
+
+* Visit/CCD based observations
 * Transmission curves for each filter, preferably for each CCD
 * MODTRAN4 is required to generate new atmosphere tables.
 * Atmosphere tables for the following telescope locations are included:
@@ -30,7 +36,7 @@ Installing the `fgcm` and `fgcmcal` Packages
 
 As of DM-16128, both `fgcm` (the third-party package) and `fgcmcal` (the stack
 interface) are distributed with `lsst_distrib`.  Therefore, with a stack
-installation after *insert weekly reference here* or v17.0 or later, setting up
+installation after `w_2018_47` or v17.0 or later, setting up
 `fgcmcal` is as simple as:
 
 ```
@@ -47,4 +53,4 @@ FGCM Cookbook
 -------------
 
 Please see [the FGCM Cookbook](cookbook/README.md) for doing a test run on HSC
-RC data on `lsst-dev`.
+RC data on `lsst-dev` and to learn the control flow of `fgcmcal`.