Add a changelog for the current features of stdlib (#537)

Author: awvwgk

CHANGELOG.md

@@ -0,0 +1,93 @@
+# Unreleased
+
+- new module `stdlib_ascii`
+  [#32](https://github.com/fortran-lang/stdlib/pull/32)
+- new module `stdlib_bitsets`
+  [#239](https://github.com/fortran-lang/stdlib/pull/239)
+  - new derived types `bitset_64` and `bitset_large`
+  - new abstract base class `bitset_type`
+- new module `stdlib_error`
+  [#53](https://github.com/fortran-lang/stdlib/pull/53)
+- new module `stdlib_io`
+  - new procedures `loadtxt` and `savetxt`
+    [#23](https://github.com/fortran-lang/stdlib/pull/23)
+    [#37](https://github.com/fortran-lang/stdlib/pull/37)
+  - new procedure `open`
+    [#71](https://github.com/fortran-lang/stdlib/pull/71)
+    [#77](https://github.com/fortran-lang/stdlib/pull/77)
+- new module `stdlib_kinds`
+  [#63](https://github.com/fortran-lang/stdlib/pull/63)
+- new module `stdlib_linalg`
+  - new procedures `diag`, `eye` and `trace`
+    [#170](https://github.com/fortran-lang/stdlib/pull/170)
+  - new procedures `linspace` and `logspace`
+    [#420](https://github.com/fortran-lang/stdlib/pull/420)
+  - new procedure `outer_product`
+    [#432](https://github.com/fortran-lang/stdlib/pull/432)
+  - new procedure `arange`
+    [#480](https://github.com/fortran-lang/stdlib/pull/480)
+- new module `stdlib_logger`
+  - new derived type: `logger_type`
+    [#228](https://github.com/fortran-lang/stdlib/pull/228)
+    [#261](https://github.com/fortran-lang/stdlib/pull/261)
+- new module `stdlib_math`
+  - new procedure `clip`
+    [#355](https://github.com/fortran-lang/stdlib/pull/355)
+- new module `stdlib_optval`
+  [#73](https://github.com/fortran-lang/stdlib/pull/73)
+  [#96](https://github.com/fortran-lang/stdlib/pull/96)
+  [#139](https://github.com/fortran-lang/stdlib/pull/139)
+- new module `stdlib_quadrature`
+  - new procedures `trapz`, `trapz_weights`, `simps` and `simps_weights`
+    [#146](https://github.com/fortran-lang/stdlib/pull/146)
+  - new procedures `gauss_legendre`, `gauss_legendre_lobatto`
+    [#313](https://github.com/fortran-lang/stdlib/pull/313)
+- new module `stdlib_sorting`
+  - new procedures `sort`, `ord_sort` and `sort_index`
+    [#408](https://github.com/fortran-lang/stdlib/pull/408)
+- new module `stdlib_specialfunctions`
+  - new procedures `legendre` and `dlegendre`
+    [#313](https://github.com/fortran-lang/stdlib/pull/313)
+- new module `stdlib_stats`
+  - new procedure `mean`
+    [#124](https://github.com/fortran-lang/stdlib/pull/124)
+    [#130](https://github.com/fortran-lang/stdlib/pull/130)
+    [#132](https://github.com/fortran-lang/stdlib/pull/132)
+  - new procedure `var`
+    [#144](https://github.com/fortran-lang/stdlib/pull/144)
+  - new procedure `moment`
+    [#153](https://github.com/fortran-lang/stdlib/pull/153)
+  - new procedure `corr`
+    [#191](https://github.com/fortran-lang/stdlib/pull/191)
+  - new procedure `median`
+    [#426](https://github.com/fortran-lang/stdlib/pull/426)
+- new module `stdlib_stats_distribution_PRNG`
+  [#271](https://github.com/fortran-lang/stdlib/pull/271)
+- new module `stdlib_string_type`
+  - new derived types `string_type`
+    [#320](https://github.com/fortran-lang/stdlib/pull/320)
+  - new procedure `move`
+    [#467](https://github.com/fortran-lang/stdlib/pull/467)
+- new module `stdlib_stringlist_type`
+  - new derived types `stringlist_type` and `stringlist_index_type`
+    [#470](https://github.com/fortran-lang/stdlib/pull/470)
+- new module `stdlib_strings`
+  - new procedure `to_string`
+    [#444](https://github.com/fortran-lang/stdlib/pull/444)
+  - new procedures `strip` and `chomp`
+    [#343](https://github.com/fortran-lang/stdlib/pull/343)
+  - new procedures `starts_with` and `ends_with`
+    [#384](https://github.com/fortran-lang/stdlib/pull/384)
+  - new procedure `slice`
+    [#414](https://github.com/fortran-lang/stdlib/pull/414)
+  - new procedure `find`
+    [#433](https://github.com/fortran-lang/stdlib/pull/433)
+  - new procedure `replace_all`
+    [#436](https://github.com/fortran-lang/stdlib/pull/436)
+  - new procedures `padl` and `padr`
+    [#441](https://github.com/fortran-lang/stdlib/pull/441)
+  - new procedure `count`
+    [#453](https://github.com/fortran-lang/stdlib/pull/453)
+- new module `stdlib_system`
+  - new procedure `sleep`
+    [#54](https://github.com/fortran-lang/stdlib/pull/54)