Added documentation on the various profiling toolsview-issue-performance

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+# Profiling
+
+To make it easier to track down performance problems GitLab comes with a set of
+profiling tools, some of these are available by default while others need to be
+explicitly enabled.
+
+## rack-mini-profiler
+
+This Gem is enabled by default in development only. It allows you to see the
+timings of the various components that made up a web request (e.g. the SQL
+queries executed and their execution timings).
+
+## Bullet
+
+Bullet is a Gem that can be used to track down N+1 query problems. Because
+Bullet adds quite a bit of logging noise it's disabled by default. To enable
+Bullet, set the environment variable `ENABLE_BULLET` to a non-empty value before
+starting GitLab. For example:
+
+    ENABLE_BULLET=true bundle exec rails s
+
+Bullet will log query problems to both the Rails log as well as the Chrome
+console.
+
+## ActiveRecord Query Trace
+
+This Gem adds backtraces for every ActiveRecord query in the Rails console. This
+can be useful to track down where a query was executed. Because this Gem adds
+quite a bit of noise (5-10 extra lines per ActiveRecord query) it's disabled by
+default. To use this Gem you'll need to set `ENABLE_QUERY_TRACE` to a non empty
+file before starting GitLab. For example:
+
+    ENABLE_QUERY_TRACE=true bundle exec rails s
+
+## rack-lineprof
+
+This is a Gem that can trace the execution time of code on a per line basis.
+Because this Gem can add quite a bit of overhead it's disabled by default. To
+enable it, set the environment variable `ENABLE_LINEPROF` to a non-empty value.
+For example:
+
+    ENABLE_LINEPROF=true bundle exec rails s
+
+Once enabled you'll need to add a query string parameter to a request to
+actually profile code execution. The name of the parameter is `lineprof` and
+should be set to a regular expression (minus the starting/ending slash) used to
+select what files to profile. To profile all files containing "foo" somewhere in
+the path you'd use the following parameter:
+
+    ?lineprof=foo
+
+Or when filtering for files containing "foo" and "bar" in their path:
+
+    ?lineprof=foo|bar
+
+Once set the profiling output will be displayed in your terminal.