Add documentation for uniquecols example (#115)
Add documentation for the uniquecols examples for the 2.1 branch.
diff --git a/docs/uniquecols.md b/docs/uniquecols.md
index 46b6a30..714fcb2 100644
--- a/docs/uniquecols.md
+++ b/docs/uniquecols.md
@@ -17,7 +17,74 @@
# Apache Accumulo Unique Columns example
The UniqueColumns examples ([UniqueColumns.java]) computes the unique set
-of columns in a table and shows how a map reduce job can directly read a
-tables files from HDFS.
+of column family and column qualifiers in a table. It also demonstrates
+how a mapReduce job can directly read a tables files from HDFS.
+
+Create a table and add rows that all have identical column family and column
+qualifiers.
+
+```
+$ /path/to/accumulo shell -u username -p secret
+username@instance> createnamespace examples
+username@instance> createtable examples.unique
+username@instance> examples.unique> insert row1 fam1 qual1 v1
+username@instance> examples.unique> insert row2 fam1 qual1 v2
+username@instance> examples.unique> insert row3 fam1 qual1 v3
+```
+
+Exit the Accumulo shell and run the uniqueColumns mapReduce job against
+this table. Note that if the output file already exists in HDFS, it will
+need to be deleted.
+
+```
+$ ./bin/runmr mapreduce.UniqueColumns --table examples.unique --reducers 1 --output /tmp/unique
+```
+
+When the mapReduce job completes, examine the output.
+
+```
+$ hdfs dfs -cat /tmp/unique/part-r-00000
+cf:fam1
+cq:qual1
+```
+
+The output displays the unique column family and column qualifier values. In
+this case since all rows use the same values, there are only two values output.
+
+Note that since the example used only one reducer all output will be contained
+within the single `part-r-00000` file. If more than one reducer is used the output
+will be spread among various `part-r-xxxxx` files.
+
+Go back to the shell and add some additional entries.
+
+```text
+$ /path/to/accumulo shell -u username -p secret
+username@instance> table unique
+username@instance example.unique> insert row1 fam2 qual2 v2
+username@instance example.unique> insert row1 fam3 qual2 v2
+username@instance example.unique> insert row1 fam2 qual2 v2
+username@instance example.unique> insert row2 fam2 qual2 v2
+username@instance example.unique> insert row3 fam2 qual2 v2
+username@instance example.unique> insert row3 fam3 qual3 v2
+username@instance example.unique> insert row3 fam3 qual4 v2
+```
+
+Re-running the command will now find any additional unique column values.
+
+```text
+$ hdfs dfs -rm -r -f /tmp/unique
+$ ./bin/runmr mapreduce.UniqueColumns --table examples.unique --reducers 1 --output /tmp/unique
+$ hdfs dfs -cat /tmp/unique/part-r-00000
+cf:fam1
+cf:fam2
+cf:fam3
+cq:qual1
+cq:qual2
+cq:qual3
+cq:qual4
+```
+
+The output now includes the additional column values that were added during the last batch of inserts.
+
[UniqueColumns.java]: ../src/main/java/org/apache/accumulo/examples/mapreduce/UniqueColumns.java
