Add activerecord-explain-analyze gem

This gem allows you to get the `EXPLAIN ANALYZE` query plan, directly
from the Rails console.

The gem is installed with `require: false`, but if it was loaded on
launch, this would be it's memory load:

```
TOP: 145.3086 MiB
  rails/all: 22.4844 MiB
  ...
  activerecord-explain-analyze: 2.9648 MiB
    active_record/connection_adapters/postgresql_adapter: 2.9648 MiB
      pg: 2.9648 MiB
        pg_ext: 2.9648 MiB
  ...
```

1 files changed, 29 insertions, 0 deletions

diff --git a/doc/development/understanding_explain_plans.md b/doc/development/understanding_explain_plans.md
index 0abd86d4b4c..2ef8b3148e4 100644
--- a/doc/development/understanding_explain_plans.md
+++ b/doc/development/understanding_explain_plans.md
@@ -654,6 +654,35 @@ and related tools such as:
 - <https://explain.depesz.com/>
 - <http://tatiyants.com/postgres-query-plan-visualization/>
 
+## Producing query plans
+
+There are a few ways to get the output of a query plan. Of course you
+can directly run the `EXPLAIN` query in the `psql` console, or you can
+follow one of the other options below.
+
+### Rails console
+
+Using the [`activerecord-explain-analyze`](https://github.com/6/activerecord-explain-analyze)
+you can directly generate the query plan from the Rails console:
+
+```ruby
+pry(main)> require 'activerecord-explain-analyze'
+=> true
+pry(main)> Project.where('build_timeout > ?', 3600).explain(analyze: true)
+  Project Load (1.9ms)  SELECT "projects".* FROM "projects" WHERE (build_timeout > 3600)
+  ↳ (pry):12
+=> EXPLAIN for: SELECT "projects".* FROM "projects" WHERE (build_timeout > 3600)
+Seq Scan on public.projects  (cost=0.00..2.17 rows=1 width=742) (actual time=0.040..0.041 rows=0 loops=1)
+  Output: id, name, path, description, created_at, updated_at, creator_id, namespace_id, ...
+  Filter: (projects.build_timeout > 3600)
+  Rows Removed by Filter: 14
+  Buffers: shared hit=2
+Planning time: 0.411 ms
+Execution time: 0.113 ms
+```
+
+### Chatops
+
 GitLab employees can also use our chatops solution, available in Slack using the
 `/chatops` slash command. You can use chatops to get a query plan by running the
 following: