add mermaid graph

arc-mermaid.md

+```mermaid
+flowchart TD
+    classDef investigationStyle fill:#6c7885,color:#2d3e50,fontWeight:bold;
+    classDef studyStyle fill:#62d4c1,color:#2d3e50,fontWeight:bold;
+    classDef assayStyle fill:#ffd34d,color:#2d3e50,fontWeight:bold;
+    classDef processStyle fill:#D46275,color:#2d3e50;
+    id_0["Traits linked to natural variation of sulfur content in Arabidopsis thaliana"]
+    class id_0 investigationStyle;
+    id_0-->STUDY_id_1
+    class STUDY_id_1 studyStyle;
+    subgraph STUDY_id_1[Study: AccessionSelection]
+    end
+    id_0-->STUDY_id_2
+    class STUDY_id_2 studyStyle;
+    subgraph STUDY_id_2[Study: PlantMaterialAndGrowthConditions]
+        id_3[SeedSterilization]
+        class id_3 processStyle;
+        id_4[Stratification]
+        class id_4 processStyle;
+        id_5[Etiolation]
+        class id_5 processStyle;
+        id_6[Growth]
+        class id_6 processStyle;
+    end
+    subgraph ASSAY_id_7[Assay: AssessmentOfPlantResistanceToPseudomonasSyringae]
+        id_8[CulturePreparation]
+        class id_8 processStyle;
+        id_9[Infiltration]
+        class id_9 processStyle;
+        id_10[Measurement]
+        class id_10 processStyle;
+    end
+    class ASSAY_id_7 assayStyle;
+    subgraph ASSAY_id_11[Assay: Co-cultivationWithBurkholderiaGlumae]
+        id_8[CulturePreparation]
+        class id_8 processStyle;
+        id_12[Inoculation]
+        class id_12 processStyle;
+    end
+    class ASSAY_id_11 assayStyle;
+    subgraph ASSAY_id_13[Assay: DeterminationOfBacterialTiter]
+        id_14[DNAextraction]
+        class id_14 processStyle;
+        id_15[qPCR]
+        class id_15 processStyle;
+    end
+    class ASSAY_id_13 assayStyle;
+    subgraph ASSAY_id_16[Assay: MetaboliteAnalysis]
+        id_17[Anions]
+        class id_17 processStyle;
+        id_18[Thiols]
+        class id_18 processStyle;
+        id_19[Glucosinolates]
+        class id_19 processStyle;
+        id_20[Camalexin]
+        class id_20 processStyle;
+    end
+    class ASSAY_id_16 assayStyle;
+    subgraph ASSAY_id_21[Assay: RNAIsolationAndExpressionAnalysis]
+        id_22[RNAIsolation]
+        class id_22 processStyle;
+        id_23[SamplePreparation]
+        class id_23 processStyle;
+        id_24[RT-PCR]
+        class id_24 processStyle;
+    end
+    class ASSAY_id_21 assayStyle;
+    subgraph ASSAY_id_25[Assay: StatisticalAnalysis]
+    end
+    class ASSAY_id_25 assayStyle;
+    subgraph ASSAY_id_26[Assay: SulfateUptake]
+        id_27[SulfateTreatment]
+        class id_27 processStyle;
+        id_28[Sampling]
+        class id_28 processStyle;
+    end
+    class ASSAY_id_26 assayStyle;
+    id_3-->|1|id_4
+    id_4-->|1|id_5
+    id_4-->|1|id_6
+    id_5-->|1|id_6
+    id_6-->|1|id_9
+    id_6-->|1|id_12
+    id_6-->|1|id_17
+    id_6-->|1|id_18
+    id_6-->|1|id_19
+    id_6-->|1|id_22
+    id_6-->|1|id_27
+    id_8-->|1|id_9
+    id_9-->|1|id_10
+    id_12-->|1|id_14
+    id_12-->|2|id_20
+    id_14-->|1|id_15
+    id_22-->|1|id_23
+    id_23-->|1|id_24
+    id_27-->|1|id_28
+
+```