README.md, add about special values

IEEE 754-2008 and booleans

Author: mkgrgis

README.md

@@ -402,6 +402,11 @@ Limitations
 - For `sum` function of SQLite, output of `sum(bigint)` is `integer` value. If input values are big, the overflow error may occurs on SQLite because it overflow within the range of signed 64bit. For PostgreSQL, it can calculate as over the precision of `bigint`, so overflow does not occur.
 - SQLite promises to preserve the 15 most significant digits of a floating point value. The big value which exceed 15 most significant digits may become different value after inserted.
 - SQLite does not support `numeric` type as PostgreSQL. Therefore, it does not allow to store numbers with too high precision and scale. Error out of range occurs.
+- SQLite does not support special values for IEEE 754-2008 numbers stored in SQLite `real` [affinity](https://www.sqlite.org/datatype3.html) in opposite PostgreSQL with full support of special values. This values are `NaN` ≡ ` 0.0 / 0.0 `, `+Infinity` ≡ +∞ ≡ ` 1.0 / 0.0 `,`-Infinity` ≡ -∞ ≡ ` -1.0 / 0.0 `. All this special calculating expressions gives `NULL` in SQLite in opposite to full-supported special values in PostgreSQL. `sqlite_fdw` can case insensetive read special values for this pseudonumbers from data with `text` affinity in SQLite such as `NaN`, `nan`, `Nan`, `+INF`, `Infinity`, `-Inf` etc and convert it to special values in PostgreSQL. But during `SELECT ... WHERE ... = double precision 'naN'` or other query reverse conversion `sqlite_fdw` uses **only** standard PostgreSQL literals: `NaN`, `-Infinity`, `Infinity`, not original strings from `WHERE`. *This can caused selecting issues*.
+
+### Boolean values
+- `sqlite_fdw` boolean values support exists only for `bool` columns in foreign table. SQLite documentation recommends to store boolean as value with `integer` [affinity](https://www.sqlite.org/datatype3.html). `NULL` isn't converted, 1 converted to `true`, all other `NON NULL` values converted to `false`. During `SELECT ... WHERE condition_column` condition converted only to `condition_column = 1` and `SELECT ... WHERE NOT condition_column` converted only to `condition_column != 1`.
+- `sqlite_fdw` provides limited support of boolean values if `bool` column in foreign table mapped to SQLite `text` [affinity](https://www.sqlite.org/datatype3.html). Case insensetive values `t`, `true`, `y`, `yes` converted to `true` value, all other `NOT NULL` values converted to `false`. De facto `SELECT ... WHERE condition_column` queries isn't applicable to this case, because search condition for `bool` column coverted by `sqlite_fdw` **only** to `condition_column = 1`. *This can caused selecting issues*.
 
 Tests
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