fix: add reference comments, etc.

to code & documentation

- add references for EU character mappings
  - mobilexag/cordova-sqlite-evplus-ext-free#21
  - storesafe/cordova-sqlite-evcore-extbuild-free#50 (comment)
- update documentation of known issue with SELECT LOWER_EU(9e999) &
  SELECT LOWER_EU(-9e999)

README.md

@@ -42,6 +42,11 @@ int main() {
   - `cc test.c sqlite3.c sqlite3_eu.c -o test`
   - `./test`
 
-## Known quirks
+## Known mapping issue
 
 - Statements such as `SELECT LOWER_EU(9e999)` return results like `ınf`
+
+## References
+
+- https://github.com/mobilexag/cordova-sqlite-evplus-ext-free/issues/21
+- https://github.com/storesafe/cordova-sqlite-evcore-extbuild-free/issues/50#issuecomment-498858058

sqlite3_eu.c

@@ -40,6 +40,9 @@ void init_map() {
     }
   }
 
+  // references:
+  // - https://github.com/mobilexag/cordova-sqlite-evplus-ext-free/issues/21
+  // - https://github.com/storesafe/cordova-sqlite-evcore-extbuild-free/issues/50#issuecomment-498858058
   EU_MAP_ENTRY(0x00E4, 0x00C4);
   EU_MAP_ENTRY(0x00E0, 0x00C0);
   EU_MAP_ENTRY(0x00E1, 0x00C1);
@@ -74,6 +77,9 @@ void init_map() {
   EU_MAP_ENTRY(0x00ED, 0x00CD);
   EU_MAP_ENTRY(0x00EE, 0x00CE);
   EU_MAP_ENTRY(0x00EF, 0x00CF);
+  // XXX TODO: NEED ONE-WAY MAPPING FOR U+0131 ->  U+0049
+  // TO AVOID KNOWN ISSUE WITH SELECT LOWER_EU(9e999)
+  // OR SELECT LOWER_EU(-9e999)
   EU_MAP_ENTRY(0x0131, 0x0049);
   EU_MAP_ENTRY(0x012B, 0x012A);
   EU_MAP_ENTRY(0x012F, 0x012E);