fix(readme): add c implementation for @stdlib/stats/base/dists/invgamma/pdf

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Author: DhruvArvindSingh

lib/node_modules/@stdlib/stats/base/smean/README.md

@@ -175,6 +175,93 @@ console.log( v );
 
 <!-- /.examples -->
 
+<!-- C interface documentation. -->
+
+* * *
+
+<section class="c">
+
+## C APIs
+
+<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
+
+<section class="intro">
+
+</section>
+
+<!-- /.intro -->
+
+<!-- C usage documentation. -->
+
+<section class="usage">
+
+### Usage
+
+```c
+#include "stdlib/stats/base/dists/invgamma/pdf.h"
+```
+
+#### stdlib_base_dists_invgamma_pdf( x, alpha, beta )
+
+Evaluates the probability density function (PDF) for an inverse gamma distribution with shape parameter `alpha` and scale parameter `beta` at a value `x`.
+
+```c
+double out = stdlib_base_dists_invgamma_pdf( 2.0, 0.5, 1.0 );
+// returns ~0.121
+```
+
+The function accepts the following arguments:
+
+-   **x**: `[in] double` value parameter.
+-   **alpha**: `[in] double` shape parameter.
+-   **beta**: `[in] double` rate parameter.
+
+```c
+double stdlib_base_dists_invgamma_pdf( const double x, const double alpha, const double beta );
+```
+
+</section>
+<!-- /.usage -->
+<!-- C API usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+<section class="notes">
+</section>
+<!-- /.notes -->
+<!-- C API usage examples. -->
+<section class="examples">
+### Examples
+```c
+#include "stdlib/stats/base/dists/invgamma/pdf.h"
+#include "stdlib/constants/float64/eps.h"
+#include <stdlib.h>
+#include <stdio.h>
+static double random_uniform( const double min, const double max ) {
+    double v = (double)rand() / ( (double)RAND_MAX + 1.0 );
+    return min + ( v*(max-min) );
+}
+int main( void ) {
+    double alpha;
+    double beta;
+    double x;
+    double y;
+    int i;
+    for ( i = 0; i < 25; i++ ) {
+        x = random_uniform( 0.0, 10.0 ) + STDLIB_CONSTANT_FLOAT64_EPS;
+        alpha = random_uniform( 0.0, 10.0 ) + STDLIB_CONSTANT_FLOAT64_EPS;
+        beta = random_uniform( 0.0, 10.0 ) + STDLIB_CONSTANT_FLOAT64_EPS;
+        y = stdlib_base_dists_invgamma_pdf( x, alpha, beta );
+        printf( "x: %lf, α: %lf, β: %lf, Pdf(X;x,α,β): %lf\n", x, alpha, beta, y );
+    }
+}
+```
+
+</section>
+
+<!-- /.examples -->
+
+</section>
+
+<!-- /.c -->
+
 <!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->
 
 <section class="related">