Merge ~xavpaice/charm-landscape-tagger:docs into charm-landscape-tagger:master

[Xav Paice]

Update README.md

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diff --git a/README.md b/README.md
index 03182df..748a892 100644
--- a/README.md
+++ b/README.md
@@ -6,8 +6,25 @@ This charm provides a subordinate to accept tags from landscape-client
 charms via relation data, and add them to the computer registration in
 Landscape.  Relies on being related to a LDS server unit.
 
+Because of the reliance on Landscape server, the charm is usable only on
+platforms supported by this charm, at this stage just Bionic.
+
 ## Usage
 
+This charm is intended to be used as a subordinate charm to Landscape server
+units.  The charm uses the Landscape API to update tags associated with each
+related computer, however in order to do that it does need some information
+provided by configuration.  The URL for the API, plus the API command line tool,
+are provided by the Landscape packages installed by Landscape server.
+
+To use the charm, you will need to know the API key and secret, which is
+provided by the `bootstrap` action in the Landscape charm.  Alternately, if you
+did not record that information, it is available in the GUI.
+
+The list of tags to add to computers is provided by the landscape-client charm,
+and as such we must relate each landscape-client application to
+landscape-tagger.
+
 ```
 juju deploy --to landscape-server/0 landscape-tagger --config api-key=$APIKEY
 --config api-secret=$APISECRET
@@ -16,8 +33,12 @@ juju relate landscape-client:tags landscape-tagger:tags
 
 ## Relations
 
-The 'tags' relation is used to provide a list of tags plus the hostname to the
-'landscape-tagger' application.
+The 'tags' relation interface is used to provide a list of tags plus the
+hostname to the 'landscape-tagger' application.
 
-## Contributing
+## Prerequisite branches
 
+Change
+https://code.launchpad.net/~xavpaice/landscape-client-charm/add_tags/+merge/407578
+needs to be merged in order to use this charm.  If that change is not yet
+released, you may use cs:~bootstack-charmers/landscape-client