Add additional GQL styleguide from ETSource README

docs/contrib/authoring-gqueries.md

@@ -12,7 +12,12 @@ Gqueries are in fact stored [GQL](gql.md) (Graph Query Language) procedures that
 - Base gqueries have a consistent set of base units, for example `kg` for **emissions**, `MW` for **capacity** and `MJ` for **energy**.
 - Base gqueries have a consistent nomenclature: for example **type**, **subtype**, **sector**, **subsector**, **carrier**.
 - Base gqueries should have a description.
-- To enhance readability, the preferred indentation style for gqueries is first to indent with 4 spaces, followed by an additional 2 spaces for each subsequent level (see the example below).
+
+## Styling
+
+### Use (2 spaces) indentation
+
+To enhance readability, the preferred indentation style for gqueries is first to indent with 4 spaces, followed by an additional 2 spaces for each subsequent level (see the example below).
 
 Example for `final_demand_energetic_industry_steel_wood_pellets`:
 ```
@@ -36,3 +41,94 @@ Example for `final_demand_energetic_industry_steel_wood_pellets`:
     )
 - unit = PJ
 ```
+
+#### Ruby specific
+
+Since GQLv3, you can use Ruby syntax to the GQL-statements as well, such as
+
+    1+1
+    10 * 0.5 + 10 * 2
+
+You can also use very advanced 'features' of Ruby, such as:
+
+    (1..10).inject :+
+
+Please refrain from doing that (too much), since that *usually* makes
+the GQL-statements harder to read.
+
+#### Try to be condense, if possible
+
+In stead of:
+
+    SUM(
+      PRODUCT(
+        Q(hv_net_delta_capacity_central),
+        0.20
+      ),
+      PRODUCT(
+        Q(hv_net_delta_capacity_decentral),
+        0.25
+      )
+    )
+
+Use:
+
+    Q(hv_net_delta_capacity_central) * 0.2 + Q(hv_net_delta_capacity_decentral) * 0.25
+
+#### Use Ruby syntax on one line
+
+In stead of:
+
+    PRODUCT(1,1)
+
+Use:
+
+    1 * 1
+
+#### Use GQL syntax on multiple lines
+
+And In stead of:
+
+    Q(etflex_score_co2)
+    +
+    Q(etflex_score_costs)
+    +
+    Q(etflex_score_renewability)
+
+Use:
+
+    SUM(
+      Q(etflex_score_co2),
+      Q(etflex_score_costs),
+      Q(etflex_score_renewability)
+    )
+
+#### Use condense statements
+
+In stead of:
+
+    IF(Q(hv_net_capacity_buffer_decentral_check) > 800
+    ,
+      10
+    ,
+      20
+    )
+
+Use condense GQL-syntax:
+
+    IF((Q(hv_net_capacity_buffer_decentral_check) > 800),
+      10,
+      20
+    )
+
+So, please put the **commas** at the end of a line.
+
+#### Surround operators with spaces
+
+In stead of:
+
+    Q(area_used_for_corn_crops)/AREA(areable_land)*100
+
+Use:
+
+    Q(area_used_for_corn_crops) / AREA(areable_land) * 100