Add spec for emergency voting (#249)

* Add simple changelog

* Update x/gov/spec/01_concepts.md

Co-authored-by: Aleksandr Bezobchuk <alexanderbez@users.noreply.github.com>

Co-authored-by: Aleksandr Bezobchuk <alexanderbez@users.noreply.github.com>

x/gov/spec/01_concepts.md

@@ -137,6 +137,10 @@ For a weighted vote to be valid, the `options` field must not contain duplicate
 Quorum is defined as the minimum percentage of voting power that needs to be
 casted on a proposal for the result to be valid.
 
+### Expedited Proposals
+
+A proposal can be expedited, making the proposal use shorter voting duration and a higher tally threshold by its default. If an expedited proposal fails to meet the threshold within the scope of shorter voting duration, the expedited proposal is then converted to a regular proposal and restarts voting under regular voting conditions.
+
 ### Threshold
 
 Threshold is defined as the minimum proportion of `Yes` votes (excluding
@@ -149,6 +153,8 @@ that proposals are accepted if the proportion of `Yes` votes (excluding
 proportion of `NoWithVeto` votes is inferior to 1/3 (excluding `Abstain`
 votes).
 
+For expedited proposals, it has a higher minimum threshold as its initial parameter, set at 66.7%.
+
 ### Inheritance
 
 If a delegator does not vote, it will inherit its validator vote.