Update documentation

README.md

@@ -185,7 +185,17 @@ URL:
 This approach ensures consistency and clarity when accessing various endpoint
 types through the gateway.
 
-### Pagination
+### Custom Headers
+
+#### FHIR-Gateway-Mode
+
+##### Overview
+
+The FHIR Gateway Mode allows for custom processing of responses from the FHIR
+server. The mode is triggered by a HTTP Header sent by the client named
+`FHIR-Gateway-Mode` with a value of `list-entries`.
+
+##### Pagination
 
 Pagination is supported in fetching data from a FHIR server using the List
 endpoint. This can be useful when dealing with Resources with high volume of

plugins/src/main/java/org/smartregister/fhir/gateway/plugins/SyncAccessDecision.java

@@ -323,15 +323,17 @@ private Bundle postProcessModeListEntries(
         int prevPage = page > 1 ? page - 1 : 0; // 0 indicates no previous page
         Bundle resultBundle = fhirR4Client.transaction().withBundle(requestBundle).execute();
         if (nextPage > 0) {
-            parameters.put("_page", new String[] {String.valueOf(nextPage)});
+            parameters.put(
+                    Constants.PAGINATION_PAGE_NUMBER, new String[] {String.valueOf(nextPage)});
             String nextUrl = constructUpdatedUrl(request, parameters);
             Bundle.BundleLinkComponent nextLink = new Bundle.BundleLinkComponent();
             nextLink.setRelation(IBaseBundle.LINK_NEXT);
             nextLink.setUrl(nextUrl);
             resultBundle.addLink(nextLink);
         }
         if (prevPage > 0) {
-            parameters.put("_page", new String[] {String.valueOf(prevPage)});
+            parameters.put(
+                    Constants.PAGINATION_PAGE_NUMBER, new String[] {String.valueOf(prevPage)});
             String prevUrl = constructUpdatedUrl(request, parameters);
             Bundle.BundleLinkComponent previousLink = new Bundle.BundleLinkComponent();
             previousLink.setRelation(IBaseBundle.LINK_PREV);