added some documentation

indexes/README.md

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+## Purpose
+
+I discovered some odd behavior with MongoMapper ensure_index for *uniqueness* that I wanted to document.
+
+Bottom line, it seems that the *unique* index is picky...
+
+This works:
+
+    key :email, String, :required => true, :unique => true
+
+This fails:
+
+    User.ensure_index([[:name,1]], :unique => true)
+
+Consult the Gemfile for versions...
+
+    ruby 1.8.7 (2011-02-18 patchlevel 334) [i686-darwin10.7.0]
+
+    *** LOCAL GEMS ***
+    activesupport (3.1.3)
+    bson (1.4.0)
+    bson_ext (1.4.0)
+    builder (2.1.2)
+    bundler (1.0.21)
+    i18n (0.6.0)
+    jnunemaker-validatable (1.8.4)
+    json_pure (1.6.1)
+    mongo (1.4.0)
+    mongo_mapper (0.8.6)
+    multi_json (1.0.3)
+    plucky (0.3.8)
+    rake (0.8.7)